But I couldn’t access the server-side LAN from the client.
I tried both allowedIPs = [ "0.0.0.0/0" ]; and more restricted Ip ranges for the server on the client side.
What do I need to do?
As far as I can tell with my (admittedly still somewhat limited) understanding of nixpkgs, sysctl changes are special because they create a new kernel in the store, not modifying the currently running system. Stuff like creating new interfaces, adding new peers and so on works just fine at runtime (at least in my experience).