VPN over WISP mode plus routing traffic query

Not sure if this is possible to setup but this is the scenario.

I want to be able to connect a Mikrotik router at a premises which I don’t have control of their firewall. I want to connect the Mikrotik’s WAN via wireless (WISP mode) and then configure any VPN client on the MIkrotik to call a VPN server. Then be able to connect across the internet via the VPN tunnel, connect with the Mikrotik and any wired device plugged into the Mikrotik. Effectively tunnelling through the firewall and connecting through the VPN.

I have the WISP client mode setup and the VPN connected but cannot seem to route traffic across the VPN? Should I be be just using routing or marking certain traffic using Mangle or is it just not possible?

Thanks