REG99 "Unable to connect"

Have COMCAST Xfinity and LG Q7+ phone in an area with little-to-no TMobile cell service so I rely heavily on WiFi calling.  Unfortunately, WiFi regularly drops and fails to reconnect or connects with Reg99 error.  This is a real pain when trying to do two factor authentication with bank accounts and I can't receive an SMS text!  Often the disable/enable wifi calling suggestion works, but sometimes not.  Also, wife and I have identical phones.  It works on one phone but not on other, but not always the same.  Sometimes mine works and hers doesn't; sometime vice versa.

Very frustrating!