I am happy to state that this is no longer the case. I activated my Vivo X Fold with no issues on AT&T. After the latest October update from Vivo, Full 5G and VoLTE are working perfectly on AT&T (They previously did not function prior to this update). I will provide screenshots if you'd like. The one thing that's not working is WiFi calling, which I usually expect since AT&T is really stubborn with Wifi Calling on unlocked devices. But this phone is definitely able to work on AT&T.You CANNOT make the phone work on AT&T, period. Either ditch the phone, or change carrier (that's what I did) - those are your only options. Before switching to T-Mobile, I was having the exact same issues you have, and I worked with AT&T's advanced technical support supervisor, who was speaking directly to their network engineers about this issue.
Without going into lengthy details - AT&T network is currently incompatible with this device. They have 2 options: they can either set your device as 'PDA' in their network and provision it as such, and then data works fine, but no phone calls. Or they can set it as Next Gen 4G/5G device, which has to use "nextgenphone" APN, but unfortunately their network is configured in such a way that when they set your device as a Next Gen phone - it has to recognize the phone model or their reps have to be able to manually set it by selecting from pre-defined list of manufacturers and devices, in order to provision 4G/5G data on the device . Currently, AT&T network is not able to recognize this device, and they are unable to set the device make and model manually, because the entire 'Vivo" brand is not on their manufacturers list. You can confirm this by logging into your AT&T account and trying to change your device in the account settings online. RIght now it will show as either Unknown Device, or wrong model from different brand. If you try to change the device - AT&T site will bring up a list of manufacturers to select from, and Vivo will not be on that list.