[MovedGoalPosts]

When swapping over the device connected direct to the modem, the modem must be powered off.

Wire modem to router WAN (internet).
Wire router LAN to computer

Boot up modem and let settle
Boot up router and let settle
Boot up computer and let settle.

If you are not getting a network connection or IP address (usually 192.168.1.xxx) to the computer, you need to look at the ethernet cable, the computer's network connection settings or the router - try a different port in the router.

If you are not getting an internet connection, but are getting a connection between PC and router i.e. you can browse the router admin page in internet explorer, you need to look at the ethernet or connection between modem and router.

Once you have wired connections working, then is the time to set up the wireless side of the config.