I am having the same issue. Accidentally upgraded to OS15, immediately started having issues with apps crashing and phone was un-useable so did a total restore with the intention of restoring to my last backup (made just before the accidental upgrade). Literally hours after the upgrade, no hardware changes or anything was done with the phone at all, and now I am getting the same messages about not being able to activate and phone is unable to connect to my laptop, unable to find cell network, unable to get past setup screen from the restore. I can't even do a manual setup and get the phone working without any of my old data, the phone is literally and completely useless. Has anyone found a solution to this?!