The use another router feels like the classic "fob off" agree with Alex.
I had similar when I first hooked up to FTTP, but luckily AAISP checked their side instantly, and within minutes after reporting it I was online.
Its either the ISP, or the virtual routing side as Alex mentioned, the PC test was effectively ruling out the router. If both of these things have been checked, and they insist its the router, they probably need to work with you on it. However they may not have checked and just assumed its the router.