First make sure the the internet connection is plugged into the correct part of the router, it's usually marked WAN. Faiiling that, most router/modems have natural firewalls within them. The firewall may be denying access either to the PCs or to the external internet connection. Check the settings of the firewall. The exact way to do this differs from router to router but usually, if you go into your web browser, the address 192.168.123.254 takes you into the router's setup page. Make sure there aren't any rogue settings in the firewall. If it still doesn't work, try assigning static IP addresses to each PC, set those IP addresses in the respective PCs and that should solve the problem (note: If you havn't assigned IP addresses, then ensure that the PCs are set up to get their IP addresses from a DHCP server).