The PC MODE I know of does not use the router or network but you connect your PC to the PC Input on the TV. This can be that 15 pin VGA connection.
Did you do that?
To view my PC on the TV over the LAN would require more hardware. The cable connection is the cheapest with a wireless HDMI and such ramping up the costs.
Bob
I have just successfully connected my Samsung Wireless Router CY-SWR1100 via my PC to my 40 " Samsung Smart TV Model UA32D5500. The internet LAN connection is working and I can view whatever is on the Smart Hub.
However, I cannot view anything from my PC on my TV and there is an error signal; " CHECK SIGNAL CABLE" showing on the TV screen when I switch to PC mode.
How do I view stuff from my PC on the TV using LAN and remove this message?
PLEASE HELP !

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic