@Fishlove
Ok, then make sure you replicate the settings for the Internet connection from the original router to the Archer AX12.
Furthermore, in some cases Internet Service Providers will bind the MAC address of the original router to the Internet connection, so that only that router will be authenticated. In such a case you would need to "clone" the original router's MAC address to the Archer AX12.
Anyway, I suggest you connect a computer straight to the modem (without a router) and see if that computer can then access the Internet right away. That would tell us whether the AX12 needs to be specially configured or not.