internet is only good at night

¿A dónde vives?

California,  no during the day it's fucking pointless. I don't have any of those speeds. 

I believe it depends upon where you are and how much traffic is channeled through the tower. To know more you should use a browser and go to the web UI (192.168.12.1) from a local client. Knowing the 4G and 5G NR bands and the performance of the signals is helpful. It helps set expectations and provides clues as to why it is slow or fast. Compare the reporting of the router web UI when it is good to when it is slow. Take screen clips and store them in a document to record what you learn.I made a one page document to share with others that might not understand the information. You might find it helpful. At least you will have possibly a better understanding of how the router is working and be able to ask the right questions of a T-Mobile support engineer. If you have data about the operation and post some of the facts about the router operation there are numerous people responding to the forum that can possibly help. Some people will commiserate with you and others will try to help. It does help to contact T-Mobile support and let them know there are problems in a given location. If others on the same tower are experiencing similar behavior and they get enough calls then they tend to look closer at a tower and the operation. It does take time so patience is necessary. Been there done that. Once they identified the problems they fixed it here and it has been better than it was.

It's likely there's congestion during the day, especially if there is tower modernization or local site issues. Home internet gets lowest priority as far as traffic goes when a site is congested.

Everyone tends to get faster speeds in the off-peak hours, the time where there is little cell internet or cell phone traffic. Unfortunately, for some people like yourself, this speed drop is critical and so bad that it can make the connection unusable.

There is also probably some slowdown caused by all general internet traffic during the day, because ultimately cell tower traffic goes to the backbone of the internet on fiber.

That said, some people get less speed fluctuation. My speed can fluctuate by over 200, but that's okay because the lowest speed I get now is 90. My speed on DSL before this was 7.

Been having the same problems since February 2021 when they added 5g.  Since then we have speeds as low as .23 mps then peak at 13mps. Constant buffering no service etc worsens during peak demand.  Many hours on the phone with no solutions. Just received my third router setting up this morning hope it works but expectations are low.

Based on my research it's clear TMobile launched their home internet BEFORE they had adequate infrastructure to handle the influx of new customers.  Only thing I would suggest is demand bill credits for the months of poor service.

If enough people do this it may light a fire under their butts to add capacity. Will let you know if the new upgraded modem works