There is a similar thread on the subject VPN on guest network interferes with regular network - #8 by anon50890781 - SW help - Turris forum
If you want something bit more complex of networking (split routing), which is want you want, then you may have to educate yourself a bit about the basics of routing since neither the TO frontend Foris nor LuCI from OpenWRT provides a carefree setup for split routing out of the box.
You basically have multiple wan ifaces (one physical with the ISP and one virtual with the VPN endpoint) and the egress traffic is either globally routed through one of them (in your case it sounds now the egress traffic is globally routed via Wireguard) or you have to establish a policy for routing clients (subnets) through one of the wan.
Alternatively there also the mwan package or you establish your own script with routing rules.
But either way it would require you to understand basic routing.