Two ways to do this. You can connect your router’s internet / wan port to your PC lan port. Make sure router gets its IP by DHCP (from ICS), which is the default for a router. Then enable ICS on the Wifi adapter. Only issue with this approach is it creates double NAT. That should not be an issue for most. Anything that connects to router will have an IP like 192.168.1.X, which will be assigned by the router’s DHCP service, not ICS’s DHCP. ICS’s DHCP will give an IP address to the router, not to the router’s clients.
If you don’t like double NAT, just set your router to a static IP like 192.168.137.2 and disable router’s DHCP. Then connect a lan port from router to the ICS PC. Now anything that connects to router will get an IP assigned by ICS and will be able to access internet via ICS. Anything that connects to router will have an IP like 192.168.137.X.
If you don’t like ICS due the need for a PC running all the time, you can use two routers. One router will bridge to a Wifi using dd-wrt. The other router will connect its internet port to lan port of the bridge router.
You can use either Client or Client-bridge mode with dd-wrt. Client mode is better since you get a firewall with it.