Please verify:
You now have a PC connected directly to the server via a direct Ethernet cable from PC's Ethernet port/adapter to the Server's Ethernet port/adapter - correct? Nothing else connected.
And you wish to add a wireless router creating a network serving the PC, the server, and other wired and wireless devices?
Can you explain "compromise". Inherently I believe that you mean you do not wish to disconnect or otherwise disturb the existing PC to server Ethernet wire connection.
Most wireless routers also have at least four ports for wired Ethernet connections.
Are you able to provide a couple of simple diagrams showing what you now have and what you wish to have?
There are options available - just need to ensure that the...