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UPDATE, I am posting this everywhere because I don't think the carriers or Samsung knows what's wrong
The latest update I got to my Active2 that also broke my Watch3 set the ATT APN to 'nextgenphone' which does not seem to be working yet. I created a new APN 'Phone' like it used to have and the LTE data started again.
I just bought a new Samsung Galaxy Note 20 Ultra 5G and a Samsung Galaxy Watch 3. I am fairly technical as I've been in the tech field for some 20 years. This is a complete and utterly miserable failure on the part of Samsung AND AT&T to work together to address this issue. Having switched from an Apple configuraton as I do prefer Android I am totally shocked that such a simple issue has been allowed to affect customers for years with no solution. I am going to begin my Google posts warning folks to stay away. Buyer Beware ! I find that to be an effective method of warning people away from companies that do not address their concerns. I would advise that everyone experiencing this issue do the same. Perhaps then we will get an anwer !
It seems the solution above does work, as I was facing the same deal, and I agree that this is a stupid faux pas for AT&T and Samsung to have overlooked. I don't know if their previous fix was undone by the recent firmware update for the ECG capability, but, again, the product owners should've had this on their radar. It seems the network profile (APN) being established in the watch is "nextgenphone", which should be appropriate for LTE devices, but the profile provisioned in the network is not matching, however, the "PHONE" APN is there and matches. Once I manually created the PHONE APN in the watch and switched to it, the NumberSync process was able to successfully complete for me. Here's the list of APNs for BYOD activations if you need it: https://www.att.com/support/article/wireless/KM1062162/ .