What ISP you are using? The CS's explanation is bullshit. I think the only way you can do is to upgrade the firmware of your router, and all the rest should be ISP's problem. Call the ISP and ask to send technician to fix the problem. (Cannot get online is a serious problem that they cannot deny to not support) If the technician comes up and stands firm on not allowing using router, then you have to install a third PC and use ICS on it.