I solved this problem for myself yesterday and couldn't find the answer anywhere online (clearly stated), so here's what I did.
This also works for Plex, games, etc it's policy based routing so you can have your VPN and remote access at the same time.
This configuration is for your ssh server (ie your home PC) that has a VPN running like ProtonVPN etc and allows you to connect back to your home PC remotely either for SSH, plex, anything you're serving.
This is changing the default gateway for particular ports, so that return traffic from connection attempts doesn't go back out through the VPN.
I used this page to figure out how to do this:
http://linux-ip.net/html/tools-ip-rule.html
Add a default route to some table, arbitrarily table 10 but you can use whatever number (RTFM), also obviously needs to be your router IP
sudo ip route add default via 192.168.19.1 table 10
Add a rule for the return traffic from your ssh host
sudo ip rule add sport 22 table 10
sudo ip route flush cache
check out your fancy new rule
ip rule show
And to make this persistent across reboots, add to:
/etc/systemd/network/20-wired.network
(or whatever your file there is, add the following to the bottom. This is the same as above, just permanent)
[Route]
Gateway=192.168.19.1
Table=10
[RoutingPolicyRule]
SourcePort=22
Table=10
I have protonVPN, therefore no way to forward packets from their endpoint to my pc.
Oh gotcha, I misunderstood this post as talking about a self hosted VPN, not external provider. That explains it! :D
Proton has port forwarding anyhow…
I was surprised to learn this was a thing, impressive, however;
'the VPN app sends a request to the VPN server to open a random port'
'the active port number will change when you disconnect and reconnect the VPN.'
This will not work OOTB with Plex for example, you would need to change the port in the app every time. It becomes difficult to serve anything statically, like a XMPP server or anything that doesn't let you configure the port.
You also would need to be at home to check which port you've been assigned eg if the connection drops and you get assigned a new port, defeating the whole 'remote access' thing.