AT&T WIFI calling not working with Xfi modem/router, calls don’t go through or fail outbound

For many years we’ve had great success with AT&T wifi calling on our phones, using a third party Motorola modem with our xfinity service, and third party WIFI router.  No problems, HOURS AND HOURS of calls per day, calls coming in fine, going out fine, no stutter or hiccups.

Since moving to an xfi router (the latest, white one XB7) AT&T WiFi calling SEEMS to activate, shows up on phones, but then within a few minutes and without warning it’s off, and nothing comes in.  Calls inbound fall, never ring and outbound will fail, and IF one can get a call out it often goes out while on the call or incound/outbound audio is interrupted and other party cannot hear call.  Is there some setting on the modem/router gateway that needs to be set/enabled or protocol changed.  Or, should I just move back to our legacy DOCSIS 3.0 modem as it was before?