Double NAT doesn’t have to be on your end. An ISP CG-NAT might appear the same to Plex.
Same goes for changes to the setup… with remote access, there’s more than just you and your setup. There’s regularly reports on ISPs blocking certain communications.
Have you already verified if your forwarded port is still visible/accessible from outside your home network (e.g. using a service like canyouseeme.org), and if the public IP recognized by Plex is the same identified from outside your home network and the router‘s WAN IP address?
