I'm in the same boat. I had WiFi calling working very well on my Pixel 3XL for many months, then some time earlier in August it just stopped. Endless re-boots, turning wifi calling on/off, airplane mode, trying multiple wifi networks, re-signing up for wifi calling w/Rogers...nothing has worked to get it going again.
*#*#4636#*#* let's me toggle the wifi calling on/off, but when I look at the IMS settings it says voice over wifi is unavailable.
Super frustrating. Wifi calling worked awesome for months and there is terrible cell reception in my home office, which is where I use my phone almost exclusively.