It sounds like your router is in bridge mode or NAT got turned off (which is kind of weird because I understand the router has limited settings). It could also be something screwy with the DHCP. I assume you already tried to restart the router. If not, try that. The next step would be to try a factory reset. There potentially is PPPoE information stored on the router that could be lost, so you may want to call your ISP for guidance. If a factory reset does not work then I would ask your ISP to replace the router.
Thank's for your swift reply!
I managed to fix the problem by changing the ISP's DNS to Google's DNS (220.127.116.11/18.104.22.168) in the router's config panel which is weird...
Anyway, problem solved!
Have a nice day/night.