Re-boot

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.