Transmission port closes when connected to VPN

[Hothersale]

I have Transmission 2.82 installed and working fine on my RT-N66U running Asuswrt-Merlin, but when I connect to my VPN (Private Internet Access), Transmission reports that port 51413 is now closed.

Any idea how I can open up the port?

Apologies if this is a noob question, but I've researched the problem and haven't come up with much. I've also submitted a ticket with PIA.

Thanks in advance for any advice!

 

[Hothersale]

As a follow-up question, is anyone using Transmission on their router through a VPN successfully? (I.e., with the peer listening port open.) If so, which VPN service are you using? TIA.