Batboy may be right about the line being too short, through its very rare for this to cause a problem. One sure way to find out is to use two filters, the second pluged into the phone socket of the first. This would make for a lot of attenuation, and you should not be able to sync at all, but if you can then this indicates a very strong signal and carefull choice of router may solve the problem.
I think you might have meant me (low attenuation/short line) ?
Not sure you're right about the double filtering creating more attenuation though.
The ADSL side of a filter is 'straight through' I believe.
But, as I mentioned, I also recommend trying different routers and/or firmware.
Yes sorry I did mean you. The two filters impose a lot of attenuation if.
(1) First filter is pluged in to the telephone socket as normal.
(2) Second filter is pluged into the phone socket of the first filter.
(3) the router is connected to the router socket of second filter.
All things being equal, this should result in no sync, but if the line attenuation is very low, then the router may sync.