Routing all traffic from one interface (LAN) to another (WLAN)

Hi there,
Does anyone know of a way to route the traffic from a wired LAN connection out to the WLAN connection on the same device, and back?

More specifically, I'd like to create a VPN connection to the WLAN network.
However, the WLAN network is a mobile LTE hotspot and thus I cannot access port forwarding to open up a connection for Wireguard.
A helpful member on this subreddit provided a useful suggestion to get around this; VPN into the wired LAN connection (where I do have access to port forwarding) and then route that traffic to the WLAN since it's all connected to the same device.

I'm using a RaspberryPi4 as my device with Raspbian (linux) as the OS.

Thanks a lot for your help.