You could try to perform the master reset to unlock all the settings and return the router to its default settings. If the router has been acquired from a company which was embedded to work with a specific ISP that uses PPPoA protocol, you will not be able to remove this protection. But in other case, you can go to settings and configure your router for PPPoE protocol. Other than that, there is no known restriction for routers all around the world to block the PPPoE protocol.
