For anyone experiencing hanging T-Mobile connections, I recommend trying the R01 firmware available at the following link.
In my extensive testing with the R03 firmware, I encountered daily connection losses. The only workaround I found was resetting the modem connection using a cron scheduler (in every hour LOL). However, after switching to the R01 firmware, the issue was completely resolved. My router and modem have been running without a single interruption for 1+ months.