Please take a look into your router’s settings.
See if you can find a settings related to ‘DNS rebinding protection’
If you find it, put and exemption in there to disable it for the domain plex.direct
Also look for an input field where you can define the ‘DNS Server’ to use.
Normally, this is filled in automatically by your ISP. Set it to the IP address 8.8.8.8 instead.
Then reboot your router and after a few minutes reboot your computer as well.
Oh and don’t use Microsoft Edge as your web browser to access plex. It won’t work (at least for accessing the server in the local network)