I just got two shiny new Samsung S20 5G's. They are great, accept that the WiFi calling feature does not work. I have tried everything on the internet, everything the T-Mobile guy said and everything the Internet provide said. Is there a known issue with this? It worked fine right up until the patch last Sunday (Dec 9th.) Version G981USQU1CTKH.
They will not call on WiFi. I get the "Connect to WiFi or a mobile network to make this call" error. They worked fine right up until this patch.
As per the T-Mobile (USA) software support site, they have suspended the roll-out of their previously certified firmware, G981USQU1CTKH, some 5 days ago, which may explain your issue.
There is but I would not recommend it unless you are an experienced and competent Android user as you will be going back from Android 11 to Android 10. If you can live with it I feel sure that a new update will be out as soon as T-Mobile (USA) certifies it. Possibly by the weekend even, as the TMO issue affects all their TMO S20 models.