That's pretty much what my ISP thinks as well, that the router WAN IP is blocked and by spoofing the mac I get an unblocked IP. They couldn't find anything else to explain this.
In about a week the DHCP lease will let go and give my router a new IP and I'll try again without the spoof.
Well sorry for wasting time on this!
Though I am glad I learned about spoofing mac to get a new IP adress, it could always be useful I guess.