@Tretabyte I would start over with the router...ie Forget it via the controller, then re-adopt with your controller. That should create a fresh set of valid provisioning. Next up, test forwarding...

@Tretabyte If the ISP routers have uPNP enabled, make sure it's enabled on your Synology (this may explain additional port forwards that are needed) If that still doesn't work, then make the Synology...

@Tretabyte Probably a few things that could be going on here, so let's try to narrow things down. Also, Plex is not the simplest of apps, I suggest you debug your port forwarding issue using SSH on...

@Ofrankie Update to latest firmware, then go here and change the security to be WPA-PSK with AES only (no TKIP). If that's not an option in the latest firmware, then put up with it, or buy a newer AP.

EAP235 supports PPSK with latest firmware from the support thread on this site.

@Barefootmoses Most are not POE capable. Which specific brand / model of wifi router are you using? It's usually the switch that provides the POE power, or it's done via single injector per wire.