Remote Access To Plex Server Keeps Disconnecting

If your setup and network are still the same, it’s likely some change on the ISP side.
Does your router still get a publicly routable IPv4 address from your ISP and can you see the public port from your port forward from outside your home network (e.g. using a service like canyouseeme.org)?

As for your router config – there’s no need to forward UDP traffic… just TCP.