If you go to canyouseeme.org and test your Plex port does it show as open?
Your new ISP might have CG-NAT meaning you don't get your own IPv4 address and can't be connected to directly.
Also stream a movie from your phone while connected to cellular data and then check that stream on the Plex admin screen to see what it says. IIRC the Plex Relay is fixed at 1mbps