I didn't think ISPs could technically filter macs behind the modem. As far as the isp is concerned, they register the mac of your modem and the modem doesn't do dhcp, so it will only serve one device, a pc or router. On their network this enables your WAN access. Anything behind the routers (computers) are invisible as far as your isp is concerned because your router is doing NAT. I've never had this stated problem and I do a lot of in home support nationwide. I could be wrong, I would have just thought itd be something I've encountered in the past 3 years. If your dad has PPPoE internet service, skip to the end of this post.
If I were trying to fix this problem, I would start by powercycling the modem and pressing the reset button and holding it per the manufacturer instructions to set it back to factory defaults. I would then let it finish booting, waiting for the status lights to stabilize. On the computers, I would ensure both are set to obtain ip and dns automatically, then I would connect them to the router. Once they have received an ip address and show as connected, I would test internet access by pinging google.com... If I did not get a response, I would try to ping google via ip address to see if dns resolution was at fault. If ping by ip failed then I would be sure that there was a networking issue still, and at that point I would contact the isp to find out if restrictions on their network were causing my issue.
That said, all of this would be a waste of time if the ISP was providing PPPoE internet. You know if you have PPPoE internet when your isp gives you a username and password. If that is the case, you must have a router which supports PPPoE (wrt54g does), or you will have issues like that which you are seeing.