"Wi-Fi Calling Error"

My apartment gets very poor coverage for some reason.  I get 1~2 bars in the 1st floor (living room) and no signal at all in the basement (bedroom and office).  I used to keep the phone on WiFi calling whenever I was home, but for some reason that has become quite unstable recently.  Dropping calls and so on.  So I  changed it to "Cellular network preferred", so it only switches to WiFi calling when needed.  But when I have it setup this way, when it comes time to switch to WiFi calling, it gives me the following:

Wi-Fi Calling Error

Error: ER01 "Invalid Certificate"

Any suggestions on how to fix this?  And/or how to fix the unstable WiFi calling to begin with?

Thanks