A flashing ADSL light can mean that the modem isn't picking up the signal properly. If the light is labeled something like line, link or ADSL then make sure the modem is connected correctly, with filters on the relevant sockets. If it's plugged into a secondary socket or an extension cable, test the connection by connecting the modem into the main phone socket (usually has a split facia). If this corrects things and the ADSL light becomes continuous, the problem should be resolved.
If the problem persists, please contact the Technical Support team.
However on some models of routers status lights can also flicker or flash to indicate that data is being transferred. This is normal and just indicates that the connection is in use. This normally applies to lights such as Internet, WAN, LAN, WLAN, Wireless, or the numbers 1 through 4.