Whether you turn the mobile network off or choose wifi only is up to you. Both are a pain to keep turning off and on when you leave and hopefully the real problem can be fixed.

I had a very unstable DSL connection when I had DSL so I'm still wary of DSL. Can you get into the settings on the Westell C-90? If so, perhaps there's some QOS or similar setting you can change to prioritize the Tmobile router. Or maybe prioritize the phone and connect direct to the Westell.

Based on it seeming to work ok on wifi only - perhaps there's some interference on the channel the Tmobile router is on. Maybe it's even on the same channel as the Westell. Or your neighbors are or a cordless home phone is.