iPhone XS Max wifi-calling issues

I have two devices with same iOS versions: iPhone 7 and iPhone XS Max. I rely on Wifi calling since ATT signal is non-existent inside our home.

iPhone7 has no issues with WiFi calling (ATT Wifi symbol shows up correctly and consistently on top left corner). However, with XS Max, it rarely connects to WiFi calling mode. I periodically check whether it is in WiFi calling mode or not by going to "control center" (unlike iPhone 7, XS max only displays Wifi calling status in control center).

So, I am curious if other have experienced this issue? And more importantly, is there is work around? I am using Google mesh router. May be Apple-Google don't work well with each other :)

P.S. As regards to Apple support page on WiFi calling support, it simply says that WiFi calling will not work on some WiFi network. Without providing any details on which type of WiFi has issues with wifi calling, the info is generic and simply useless.