I'm also having activation issues on AT&T (I'm on the Next plan, coming from an XS.) I'm using the new sim that came in the phone (if memory serves correct, I've never actually swapped the sims between old/new devices, only removed old sims to trash them.) I had transferred everything from the phone via that phone2phone method, not sure what it's called, first time I haven't restored from iCloud backup, just my luck.
Anyways, the iPhone XS told me the transfer was complete, but the 11 Pro was stuck at 90% or so completion, and the power button wasn't doing anything, so I hard reset, it completed setup. However, both the XS and the 11 Pro both show zero data signal. Sigh. Hope this is resolved soon! Glad I found this thread so I know it's not only me…