WiFi calling will not consistently work on m y iPhone8 (neither will my wife's)

The lady this evening spent a lot of time telling me that the phones needs a good wifi and other devices could take up the capacity.  My wifi runs 100+ meg down and 80+ up (ATT gigafiber).  Only my wife and I live here, and she barely using her phone, no gamers, my home automation and laptop are only connected.  She also told me that the phone needed a cellular connection to an ATT tower wifi to work.

Both of these are pure baloney. I just got off a cruise ship last week and WiFi calling worked fine on an extremely slow internet connection and with the phone in airplane mode, meaning no cellular connection. Your problem could be too strong of a cellular signal preventing your phone from staying on WiFi calling. Try putting your phone in airplane mode, then turn WiFi back on. That will force your phone to stay on WiFi calling.

Yes, I know that it is incorrect.  I should have included that I have very weak to non existent cell signal.  She said that could be the reason for poor wifi calls  I have had two ATT supervisors tell me that I needed to be connected to a cellular tower for wifi to work.  Other techs have laughed at this, but I still have no/poor wifi calling.  Anything else?   

Have you tried the airplane mode suggestion?

It may take a minute or two but if it won’t go to WiFi calling and this only happens at home, it has to be some issue with your home network or ISP. Try rebooting your phone. Once my iPhone 6s would not connect to WiFi calling and a reboot fixed it. Otherwise, I’ve got no ideas.

It works sometimes, most times no and not at all for days now.  We installed new network gear a couple of months ago.  I do not believe it is our network.  We att Gigafiber, I do not believe it is our ISP, or whatever.  I had a similar issue with a new iPhone 6 a few years ago...., finally talked with someone that knew what he was doing and he changed something on the provisioning.  Worked fine for years, until the iPhone 8.  

Sorry, i should have added thank you for your help.  I am hopeful that someone can help.  It really depends on ATT, most of their technical people, I believe have been through some shallow training, but their understanding/knowledge only helps with common problems.