DLink DSL-2640B wireless connection problem

Recently I brought a DLink DSL-2640B to replace my client faulty wireless router. When I finish configure the router modem, all the thing is working fine except it can't connect to the internet. At first I thought the password is wrong, but I check with the ISP (TMNet) that my password is correct. I already try to upload another version of firmware but it still can't solve my problem. Hmm weird, then I have no choice to bring the modem back to check.