After a couple of days research I've completely rewritten my question to make it easier to answer, and hopefully clearer of what I'm looking for.
I have an Ubuntu 18.04 VPS server with a dedicated IPv4 address (we'll call it xxx.xxx.152.21) on eth0, calling the server VPS1. I have installed Tinc VPN with the address 10.8.2.1 for the VPS on interface tun1, and on the same Tinc VPN network I have a small server with the address 10.8.2.10 that I'm calling SSR1.
If I place an application server on SSR1, say listening to port 8044, what is the best way of 'linking' xxx.xxx.152.21:8044 to 10.8.2.10:8044?
SSR1 is behind a router that does not allow port forwarding which is why I've set Tinc VPN up. I've looked through tools such as netcat and ip route but not sure what is best to use or if they will function for what I'm trying to do.
UPDATE
It turns out that something was wrong with my Nginx install and a full reinstallation put the proxy_pass correct. Indeed Nginx will forward HTTP packets on to any of the network interfaces. I'm still working on a way to pass other protocol packets on different ports on to the internal network and not sure if this should be done using IPTables or netcat. My only concern about netcat is I'm not sure to what extent it means by having to start a new process with each connection/request as mentioned in the documentation.