Inconsistent speeds

It would help to make the call to T-Mobile support and ask if there is work on the equipment in the area. That sounds like they may be working on the equipment. Sure it might be the router but it is just as likely or maybe more so that it is with the equipment on the tower. When operation became unpredictable and unstable here that is what I found out and only by contacting support and asking if they were doing maintenance on the tower. They will know the PCI, physical cell id that your router connects to and can look at service logs. 

If you post the metrics from the primary and secondary internet connections from the 192.168.12.1 web interface then others can see how the signals are and offer more direct suggestions. There are things you can do to improve the performance but knowing how the router is operating and what cellular channels are servicing the router it is hard to provide much help. The cellular home internet solution is a little more effort than just plugging in a hard connected router/modem. With a little extra effort it is possible to improve things.