Cannot access local server after allowing remote access

I use a Windows 10 PC, and have recently purchased a Synology NAS DS218+, and I cannot access PMS locally after allowing remote access.

So, the problem appears like this. I have installed PMS 1.19.1.2701 on the NAS, i have set it up for movies, tv shows and home videos. All this while i am logged in to the PMS using Google Chrome going through the local ip adress and port 32400. I have, at this point, not enabled remote access (i unticked the little box during setup). However, when i do allow remote access, a port forwarding rule is automatically created:

Plex Media Server

Protocol: TCP

External Host -

Internal Host: 192.168.0.xxx

External Port: 15813

Internal Port: 32400

Options: UPnP Rule

After this, I can no longer access PMS through internalip:32400/web. I also cannot change the port forwarding rule without factory resetting my router (a Sagemcom F@ST 3890V3).

All that happens is that the error message ERR_CONNECTION_TIMED_OUT shows up on screen, together with a recommendation to check my connection, my proxy and my firewall. The connection and the firewall are both working OK, so it must be the proxy? Or is it something else?

Do I have to fake a local connection to the NAS via a SSH tunnel? That doesn’t seem like the way to go. I suspect the port forwarding rule is the problem, but I am no expert. I don’t know how to fix this, your help would be appreciated.

Server Version#: PMS 1.19.1.2701
Player Version#: web interface, DS218+