T-Mobile Home Internet Frequently Stops Working

So correct me if I am wrong but it appears you are running your testing with your iPhone 12 Pro Max. Are you sure your speed test is across the wireless LAN out the gateway or is it across the phone's cellular signal? If you want to have a solid test out the home internet gateway be sure that is the path the traffic is taking. If you have a MacBook run the Speedtest.net application from a wireless client or run the speedtest.net "test"  via a web browser on the client. If you launch speedtest.net in a browser you can enable the advanced reporting. 

If you want to get more predictable results it does help to use the same server. With speedtest.net you can use the "change server" option and then select the 3 vertical dots to get the options and select "Favorite this server". Sure you can just use whatever but results may not be very predictable. When you run a test dont run just one run several tests a few minutes apart to get an average. The results are not just about the speed. Look at the Ping millisecond response, the amount of Jitter and loss. Watch download latency and upload latency with different servers. There are several metrics to consider with the speed testing.

Really it comes down to you can test however you want. The Ookla Speedtest is probably the best to use. To understand how they work see their document.

https://resources.ookla.com/hubfs/Ookla%20Speedtest%20Methodology%202020.pdf

I am positive that I am using the Wfi on my iPhone through the T-Mobile Home Internet (when I’m using the phone’s cellular signal, I get the 5G icon on the top of my phone).  
 

Thank you for telling me how to change servers. I will pick one and use it constantly from now on. 
 

Interesting that I see the servers changing from moment to moment and are not always available for testing!

I picked the closest tower to me and am having slow response time!

The download speed is so poor. Try to record the cellular metrics when the test is good and poor. http://192.168.12.1/TMI/v1/gateway?get=all

The ping latency is high when the signal is poor and the jitter reflects erratic behavior. I would suggest to favorite the Syosset, NY server. Some others may be closer but that server seems to have more favorable response metrics. On the last test the phone only reported 2 bars on the cellular signaling. To see more about the cellular signal strength the phone records you can put your phone into FieldTestMode and see the RSRP cellular signal strength. To do so you enter *3001#12345#* then enter/send. The dashboard will report the RAT info. Select the menus bars at the top right to see 5G NR connection info. Scroll down for the LTE info and the serving cell info. The serving cell reflects the band the service is currently on. The PCI value is the physical cell I'd and the RSRP is the signal receive power. The PLMN and cell ID help to locate the serving tower on CellMapper.net if you want to determine where the tower is and how far away it is. This information helps understand how the signal is and also can be helpful when trying to improve the signal. 
The poor download speeds make me suspicious that the gateway is not holding a 5G signal and reverting to 4G LTE on the primary signal. It would be good to know if the gateway is holding both the primary and secondary signals or not. The LED screen will not tell you about that. 

The first test you posted suggest to me the 5G signal is probably the n71 band based upon the results and the number of bars on the phone cellular signal info. The phone may not have been using 5G at the time but for those results the gateway would have had to of been picking up a 5G signal on the secondary channel. If the gateway can produce those results the tower with the 5G signal is probably only 2-3 miles away at most. I suppose if the 5G band is the n41 and you are out on the outer limits of its delivery that could be the case and that might explain things. If the 5G secondary signal comes and goes the extreme speed change when testing would make sense. 

Today, not even getting 1 mps!

Come on T-Mobile!!!

Another day, more slow internet 

Just ONE mbps today!

You need to contact T-Mobile, show them the data and demand some answers. If they are working on upgrades to the system the behavior is maybe understandable but they need to provide some assurances that things will improve. The service does not come close to what they advertise. It is worse than the worst cable vendor we had 30 years ago. If they cant provide answers and some evidence that there will be resolution then it might be time to pull the plug and move on. The monthly price, no contract, and no data cap might be attractive but with the bandwidth provided most of the time you would be hard pressed to ever reach a data cap. 

The problem appears that it is service delivery. You need to press for answers. If they deliver and can provide proper service great. If not then do what you need to do.