What do you mean by "connect PCs" ?
What is this router ?
Not clear to me. But what I expect is that you have no IP address on PCs for the ethernet cable.
Whe wifi is working because of it handle a dhcp server.
Mac "connect" by using mdns or they have the same "fake" subnet due to the self assigned IP so it match.
What you should try is to check the IP address of the mac, then you set a fixe IP address to PC in the same range.
Tell us more about your config and IP of each element, it will be usefull