It is better to provide your network topology for furhter look into it. Follow the guide as below to check your configuration step by step first. Ensure configuration on both matches each other....

What's the hardware version and firmware version of your W8968? change dns to "Use user discovered DNS server only", but the save button does not work. Can you provide a screenthot? With other web...

1) With PPPoE connection, there is WAN IP address but no internet access? No need to configure any access control to get internet connection. 2) Cannot find what you said in the emulator of WR720N....

Well, I guess Windows 8 and MA260 are not completely incompatible with each other and that's why it works on your PC but the problem occurred.

You can access internet indeed when PC is connected to your modem directly, right? Can you check IP address on your PC? I'm not asking for your modem's IP.

It is brand new or was working properly? Can you login its web itnerface when it stops working? Tried reset it to factory defaults and reconfigure it? Maybe it is defective...