Port forwarding not working for plex or transmission on synology NAS

[IcaMaxi 0]

I'm running transmission in a docker, and plex as a DSM package.  I've read through a myriad of posts.

I have opened a port on https://airvpn.org/ports/ for X remotely, and 32400 locally. I've confirmed the remote port is open. On my router, I've forwarded the external port X, for all hosts, to port 32400 on my NAS. On plex, the local port is fixed at 32400, and I've set the remote port to X. Same thing on my transmission docker, only with one more layer of ports to get inside the container.

I've confirmed that I can reach the internet with the VPN turned on, and that with the VPN off, all port forwarding works fine. I can access my plex server remotely, and the transmission peer listening port reports as open.

Edit: Like an idiot, I didn't also try this on my desktop until now. Running Ubuntu 20.04, the Eddie client, and transmission, same setup as previously (with different ports, so no conflicts), and I'm still having the same problem. So obviously I'm doing something wrong, or something is wrong with my router? My router model is sagemcom 3890v3 by the way. Modified to my specific ISP, and so imposed on me. Only ISP available where I live as well so there's unfortunately no getting around that.

Anyone have any ideas on what I could do to fix this?

Thanks.

Edited ... by IcaMaxi

[OpenSourcerer 1441]

So, which device connects to AirVPN, the Sagemcom router or the Synology?

[IcaMaxi 0]

Sorry, I should have specified. The Synology NAS is the one I set up the VPN on.

[OpenSourcerer 1441]

If the NAS connects to AirVPN, setting anything on the router is superfluous. The only setting you need to take care of is to make Transmission listen on the tun0 interface. Since it's running in a container, then you probably need some option you can give to Docker to tell it to listen on tun0 instead, I don't know the details there. #readthedocs