Will using my own router resolve some issues with the T-mobile router?

I'm surprised to hear this. I don't see how adding a router to the gateway would have any effect on the speed you receive via TMHI. Only the gateway is facing the cell tower to grab the signal. All that goes on is that the gateway passes the Internet it receives to the router.

Have you tried an experiment where you measured the speed with the router connected, and then disconnected? Do it at the same time of day so you're comparing "apples with apples" and not seeing speed differences caused by different tower loads at different times of the day.

¡Gracias!