How to change Ring Cellular back-up service provider

The factory service provider for ring is AT&T, I live in a rural area and have a microcell to provide cell service, however T-Mobile has good service in my area, how do i go about changing the sim or getting a T-Mobile sim from Ring to allow cell back-up to still work, I have run into instances of the alarm being disarmed in cell back-up and still going into alarm on the monitoring side and had a couple not so pleasant visits from the local Sheriffs Office.