So I the cameras IP addresses are: 192.168.8.100 and 192.168.8.101.
But apparently what I want to do is not possible anyways.
Customer support already confirmed that it is not possible with Glinet to connect to WAN-blocked devices via Wireguard because that is also in the WAN zone or something like that.
Very inconvenient as this is a common use-case (at least for me): Block internet of sensitive devices, servers, etc. and access them via VPN.
Thus I now have to set up a dedicated wireguard server…