wi fi router for gateway

I just looked at the Ascus router (Amazon.com: ASUS AC2900 WiFi Gaming Router (RT-AC86U) - Dual Band Gigabit Wireless Internet Router, WTFast Game Accelerator, Streaming, AiMesh Compatible, Included Lifetime Internet Security, Adaptive QoS). It's only $125 on Amazon. Does this router need to be plugged into the gateway with an ethernet cable? (That I would have to drill holes in floors to route).

I switched to an Asus RT-AX86U when I got the T-Mobile internet also. I had the Google Nest mesh system prior with Comcast and it doesn't work well with T-Mobile. I spent the $250 for the Asus and it works great. My house is a single story around 2100 sq ft. I get 600mbps down in the room the router is in and around 400mbps on the other side of the house. Spend the money for a quality router and save yourself headaches from buying a cheap router.

I have 86 devices (not a typo) connected like the doorbell and 5 other cameras along with 44 smart lights, some smart switches, thermostat and a few other sensors and all of this is on the 2.4Ghz side. On the 5Ghz side I have two of the five TV's, three phones phones, three laptops, tablets, Apple TV, chrome cast and google minis.. This was my same setup with the AC-86U and the AX-86U and both handled all of this without an issue.

I can personally vouch for the Asus AC86U that the other comment and I mentioned. But I have also heard good things about some TP-Link routers. A quick search and I see it the TP-Link has QOS. I mention that since with QOS on my Asus restricting my TV's to a 15MB download and 1 up speed caps.  I am able to get four TV all playing 4K HDR videos, a 5th TV streaming 1080P, streaming spotify to 6 wireless speakers with my doorbell camera constantly streaming all the time to my NAS server.
When I ran this same test with the WiFi in the T-Mo gateway I was only able to get two TV’s with 4k videos and one with 1080P and streaming the music since it does not have QOS.

If you give some of the video streaming services and inch they will take a mile. Without that QOS I have seen netflix pull 60 to 70 bursts that messed up other things streaming on the network. I guess my point is with a little router management that has features like QOS it can defiantly help.

I also name the 2.4Ghz and 5Ghz with different names so I can designate two of the TV’s on 5 that can do 5Ghz along with the speakers and the other stuff on 2.4Ghz.
Now this was my extreme test, I will never have four TV’s all running at once with 4K movie streams.

I have had the T-Mo home internet running for two weeks now using an external router and haven't has any issued. My house is kept at 75F and I don't use a fan, but I also have the T-Mo WiFi turned off since the router manages my WiFi.
You mentioned other ISP options. Even if you have a little time to setup and manage the network on an external router with the T-Mo gateway and decide to change later its been my experience that some other ISP's come with very cheap WiFi built in and may even charge you extra for the WiFi enabled gateway. It's always easier to just move your gateway over to the new service and that way you know everything should still work just like it did on the old service.
I will admit that I only tested the built in WiFi on T-Mo's gateway for a day and ran that extreme test for an hour. The built in WiFi impressed me with the coverage and signal quality. It's much better then some other ISP's built in WiFi' units I have seen and tested over the years. But I have a feeling if I ran it longer I might have also seen some of these suspected heat issues others are reporting. My speeds have been  consistent and stable. Labor day weekend will be interesting and I might see a slow down since I live by a lake and everyone is going to be on the lake using that same tower. Maybe I should put everything back onto the T-Mo gateway wifi for this week and see what happens?

To answer the questions above.  I have a ranch house, 2100 sq, ft. I usually have my phone, laptop, a tv (streaming) and ring doorbell connected. This can go up to 2 tvs at a time and more cell phones. I don't use it for gaming, etc. Just streaming and normal computer activities. I also have a wi fi repeater, but I am not even using that right now.

I ordered the fan that Nater Tater recommends, it hasn't arrived yet.. I have the gateway in the coolest room of the house. I keep my AC at 68 so it's pretty cool in there. it is near a window but does not get direct sunlight,  

The reason I was looking for an inexpensive router at first was to see if it even helps. I am on the fence about even keeping t-mobile. I am not in a rural area. I have about 5 other choices including comcast, frontier, verizon ….. They arbitrarily changed my bill from $50 to $65 a month…….  I was without internet for almost a full month due to the first gateway dying, then the 2nd gateway they sent didn't work either. So now, this third one works, but my speeds are so slow.  I am between a bunch of towers. The tower I connect to, no matter where I put the gateway in my house, is to the northeast and according to cell mapper not even transmitting my direction. I am in range for 2 other towers, but they both appear to be not active now.  I think they are doing maintenance on 2 of the 4 towers in the area and they just have all of us jammed on one tower….

I ordered a TP-Link Wifi 6 AX1500 Smart WiFi Router (Archer AX10) - 802.11ax Router.(It is returnable) If I need to buy an ASus I can. I am disconnecting Frontier at the house I just sold and moved from. It has a brand new router (that I will need to return) but I could try that one too,  

Thanks for all the help guys. I am just a retired "old" lady. I am recovering from a broken femur and  just last week had to have foot surgery on the opposite leg. SO…..I am house bound for now, have no cable, just stream and surf on my laptop. I need some decent internet so I don't lose what's left of my mind!  Thanks again

What most people don't realize is why some routers cost more than $150 and those reasons are actually very important.  If it is going to be the only way of accessing wireless and connecting devices to the modem.  How many devices are we looking at?  Higher end routers not only have way better processors to handle the more demanding network situations without slowing down, but they also have more RAM to make sure they can accomplish all of that as well.  Personally, I use an Asus RT-AX86U, which was about $250 when it came out last year in August.  I have more than 20 devices always connected and using WiFi, some of which are AX compatible and some other hard wired stuff, including a desktop connected on the 2.5GBit port at 2.5Gbit.  This usage case scenario is a good example of why more expensive routers can be worth the price.  

Asus routers are a favorite among T-Mobile customers for a few reasons.  Asus used to partner with T-Mobile for their WiFi Calling routers, which was an RT-AC68U with custom firmware.  The Asus firmware is fairly simple and has all of the options most would want and more.  It has easy setup options for being used as a router, access Point, and Repeater.  This development is worth the premium, at least in my opinion.  There are also some great custom firmware options out there for those looking for more out of their Asus routers.  

Ultimately, how many devices you are using now and think you will have in the future should be a big part of your decision.  Future proofing might be important with WiFi 6/6e but you will pay the early adopters fee.  There are some great AC routers out there.  I have linked one on Amazon to give you an idea of what I think would be worth buying for now and still serve you well 2 years from now if you have no plans for AX/WiFi 6/6e devices over that time.  

Amazon.com: ASUS AC2900 WiFi Gaming Router (RT-AC86U) - Dual Band Gigabit Wireless Internet Router, WTFast Game Accelerator, Streaming, AiMesh Compatible, Included Lifetime Internet Security, Adaptive QoS

I would think with 3 bars on 5G you would get more then 1MB download speed.
As for the ethernet cable a Cat 5e or higher number would work.

As for a WiFi 6 router, well I also use an Asus WiFi 6 router like Nater, but not the same one. I just happened to already run wifi 6 before changing over to T-Mobile Home internet. although WiFi 6 does have some advantages I can personally say that my other Asus WiFi 5 RT-AC86U router had the same coverage for a 1800sq ft house.
Can I ask for some more details:

How big is the house/apartment you are covering with the wifi?
Is your house air conditioned, or what is the temperature in the house.
Do you have it in a window? Mine works better against the wall facing the closest 5G tower.

How many devices on the wifi?
Do you have both 5Ghz wifi and the 2.4ghz wifi running? Just asking because you can turn off one of the the 5Ghz so it runs a little cooler. But in most cases if you dont transfer large files to a home NAS or server then you would probably turn off both 5Ghz radios and just use the 2.4Ghz one then see if it runs cooler and faster. Then you might know its maybe overheating.
Any other details you can provide will be great and I will try to help.