I was having the exact same problem and found the answer on a review of the router at amazon.co.uk!
See Al's comments at this link
http://www.amazon.co.uk/NETGEAR-RP61...ustomerReviews
Switch the router on BEFORE the STB [or cable modem in my case]. Let the router settle down make sure you can reach it from your PC. THEN turn on the STB/cable modem.
Hope that does the trick for you, it worked for me.