In my current project I am trying to make a "out of bounds" Raspberry PI(Raspbian Strech) for remote troubleshooting of network equipment. To connect to the remote location in case of an outage of some sort, I use PIVPN on wlan0(4G). However, I want all other data(updates, etc...) to flow through eth0("Landline").
The VPN connection only works if wlan0 is selected as the primary interface (using metric values). If eth0 is selected as the primary interface, the VPN connection comes in through wlan0 and to its destination (192.168.20.4) but seems to exit the eth0 interface(??) thereby, failing the initial handshake. I found this out using wireshark and by logging any exiting packets matching port 1194 on eth0 in iptables.
I have attempted to use iptables, routing tables and ip rules to route all wlan0 traffic back out through wlan0, but I have had no luck.
Any ideas on how I can go about solving this issue?