Just to be thorough, Did you try Power-Cycling the MODEM as well AS the Router itself?
Many Modems must be Power-Cycled in order to release/renew the MAC Address mapping of the device directly connected to it's LAN port (IE: Router or PC)...
Just a thought,
If You're Unsure, "RTFM"... If You're SURE, "RTFM" Anyway.