OK so you have the PC wired to the router via ethernet.
I would turn everything off and unplug all , turn the modem on let it settle, turn the router on, reset the router, turn it off, plug it into the modem turn the router on, then plug the pc into the router, turn PC on, log into the router and upgrade the firmware and see if that fixes it.
What is the status of the lights on the router?
Is your PC TCP/IP settings for the LAN set to obtain IP and DNS automatically ? (see manual appendix for details)
I'm guessing the settings detected within the router should be;
VPI=0 VCI=38 PPPoA encapsulation=VC-MUX
dynamic IP address