When you have a connection problem with a cable router set-up, there is a particular method of rebooting the system which usually puts it right.
Shut down your PC.
Remove the power plugs from the router and the modem.
Leave everything powered down for 5 minutes.
While you are waiting, check the connections.
You should have an Ethernet cable connecting the modem to the WAN port on the router.
A second Ethernet cable should connect the PC to one of the router's LAN ports.
1. First power up just the modem and wait until all the lights settle down (usually about 1 minute).
2. When the modem is powered up correctly connect the power to the router and again wait until the router's lights settle and are steady.
3. Finally boot up the PC.