So my Goal is that I have a client (A RPI that i send to my customer) connect from behind a firewall to an open access server and then when I connect to the same open access server I can then connect to that network behind the firewall. I know that openaccess already supports this with the remote gateway option but you need to know the subnet the client is on to create the routes. what i want to know is there away to have the client automatically set the subnet when it joins? or is there away with OpenVPN to create a L2 VPN Bridge form the client side sort of like Softether?
Your best bet is to use the RPi as a router (NAT/masquerade, etc.) or as a “bastion” host, from which you can run commands like ssh.
Tailscale is much easier to accomplish this, but you do need to know the remote subnet and tag the Pi as the GW to the subnet.