So, i was able to set up a client VPN but i'm unable to reach the ISP router.
In fact, currently, i'm nether able to reach it via LAN interface, nor via client VPN interface.
I did a packet capture and could see a ping going in to the corresponding interface, but it didn't come out at the WAN interface.
My best guess at this point is, that i'm missing a route but: The isp router is directly connected to the WAN interface, there should be a route without defining one.
AND adding a route manually gives the error "The static LAN route "Test" has an invalid next hop IP. The IP address 192.168.178.42 is not on a configured subnet."