4g and 5g compatibility of the many LG V60 models

None of those will get 5G.  If you want a v60 that will work with ATT you need to buy a ATT branded v60

Here are the compatible unlocked phones for att

https://www.att.com/ecms/dam/att/consumer/help/pdf/Service-Capabilities-Unlocked-Devices-ATT-Network.pdf

Also, any AT&T 4g and 5g compatibility with the xiaomi mi 10 ultra? 

When I asked this question to customer service in a chat, they repeatedly told me that as long as the phone is unlocked it will work with the 5g network. They also mentioned they are working to update the website. I keep getting conflicting information. 

Yeah customer service is wrong..  if it's not on the list it won't work.

So this means that I can buy and use avn international dual sim s20 ultra and it will be able to use AT&T 5g? And I imagine the note 20 ultra will eventually be on that list too? 

No it has to be the us model hence the reason they have the model numbers listed.