This shouldn't be a big problem. Yes the 601 router needs to be connected to the cable modem using the WAN port on the back of the 601 router to the ISP modem. Be ware that some modems have built in routers themselves and can cause some connection issues. It's preferred to have an ISP modem that doen't have a built in router however not everyone can do this, especially with having phone or VoIP service packaged with Internet services.
Ensure DNS IP addresses are being filled in under Setup/Internet/Manual?
Turn off QoS options.
Turn off Advanced DNS Services if you have this option.
Turn on DNS Relay under Setup/Networking.
Setup DHCP reserved IP addresses for all devices on the router.
Ensure devices are set to auto obtain an IP address.
Set Firewall settings to Endpoint Independent for TCP and UDP.
What wireless modes are you using?
Try single mode G or mixed G and N?
What security mode are you using? WEP, WPA or WPA2? Preferred is WPA2. Auto TPIK and AES.
What wireless devices do you have connected?
Any cordless house phones?
Any other WiFi routers in the area?
Turn off all anti virus and firewall programs on PC while testing.
Turn off all devices accept for one wired PC while testing.