A year or so ago I tried the T-Mobile home internet thing. I knew that theoretically it shouldn't be great service, but I live in an area with very few options and it actually performed decent. Until one day I could not get anything to work. A restart would get some things working, then it would stop. They replaced the gateway, but same thing. I finally set up my old provider and then found an article online that actually helped fixed everything for T-Mobile.
I have always believed in separate modems/routers, but they advertise that "gateway" as an all-in-one and say not to connect it to a router. But when I connected it to a router and just used the gateway as a modem, everything started working fine. The same thing worked for my coworker. If you have an old router sitting around, I would try plugging that gateway into the WAN port of the router.