Belkin is correct in its assessment - your ADSL signal is not quite strong enough for the router. You must remember that modem/routers tend to be more sensitive to gain deficiencies and other line problems than conventional modems.
You have not mentioned whether the RJ11 goes into the main BT socket, an approved extension or a DIY extension. This is something that you need to look at carefully. You may also like to make a small modification that bypasses the old redundant bell circuit in the main socket, which will improve the gain. Link to follow.