Re-boot

I was interested to see your post. I see an improvement after I reboot my modem but was afraid it was just wishful thinking.

I don't want to have to keep doing this, though. I'm paying a pretty penny for my data plan, and am furious when it's so slow I can barely do anything.

Not sure if this is your case, but here's a possibility. There's some guesswork and assumptions I'm making about how the service works.

When you reboot the gateway, I believe it will search and lock on to the fastest/best band/tower connection it can find at the time. If during the day that connection degrades or disconnects, the gateway will switch to a different tower or band. As long as that new connection stays stable it will stay with it even if the better band/tower becomes available again. The only way to get back to the ideal band/tower is to reboot assuming the preferred band/tower is available, of course.

I occasionally have issues with the 4G (Primary) Signal switching to a different band/tower. This is usually only a minor problem though. But, a few times, I've noticed my 5G (secondary connection has switched from the n41 band/tower to the n71 one which is much slower. When I reboot the gateway it goes back to the best signals.

So, before you reboot next time, open up the T-Mobile Home Internet app, select the "MORE" tab, then "Advanced Cellular Metrics". Take note of the information for both the LTE and 5G signals. Then once you've rebooted, check the metrics again and see if there's a substantial difference and if the band is different. There are discussions throughout this community about what the metrics mean.

If it is switching and it's a constant issue for you, you may need to contact T-Mobile and see if they can do anything about it. Good chance they'll just tell you they are "working on the towers" so be prepared for that.

I've been going through the same thing. But I also experienced that with some other routers, like Unifi, so I added a "smart plug", which detects when the router loses connection and does a controlled power cycle. There's several different ones available, and I've used a few…but this one was the easiest and works the best for me. You can even schedule resets instead...or set multiples up in a master/slave configuration. 
I wish it was a smaller form factor, or had a built-in pigtail because I have to plug mine into a power strip, so I had to buy one of those little 4” long extension cables so it doesn’t block 3 outlets.

https://tinyurl.com/Keep-smart-plug

https://tinyurl.com/outlet-savers

If you're a bit more of an advanced user, I really like this next option, because they can provide a REST API, and you can do lots of things with it using HTTP commands. Also has a cloud interface with a mobile app, bluetooth connection if you prefer, and a little built-in webserver.

https://tinyurl.com/super-duper-router-manager
 

I hope this helps relieve the same level of stress and disappointment that I experienced. And for transparency, I have no relationship whatsoever with any of the vendors or. manufacturers of these items I've linked...I bought them with my money (along with several others that I returned), and am just sharing my experience using them.

T-Mobile, if you're watching, perhaps consider adding a watchdog feature like this to your gateways. It actually is a very simple thing to implement internally if you monitor signal strength and if it drops below a threshold, open the 5V circuit, set a 3000-5000 ms timer, and re-close the 5V circuit. This can be done with a little cmos battery.