1. connect the pc to the modem and test to if you can connect to the same site.
2. if you can not, sth is wrong between att & china telecom. call your isp.
3. if you can connect with the modem, check the router mannual to see if there're options to manipulate TTL or session timer (not sure) or try another router ($9.00).
I might be able to check your router for you when I have time if you can post its vendor and model here.
Have a nice weekend.