Same problem, no solution :(
I've bought two Sonoff RF units with remotes. Both connect to WiFi ok, BUT neither pair with 433MHz remote using instructions provided. ie. press button twice rapidly to place in pairing mode. No red LED and no pairing. I tried pressing button on RF board for 2 sec (as per post by Mike), but still no result. I note there has been no response from ITEAD - this is very frustrating!! I have attached a photo of the unit I have. CAN SOMEONE FROM ITEAD PLEASE RESPOND.
I have the same problems with 2 sonoff RF, Its nor pair at all, with the original remote that sale in Itead