Presumably you mean the cable that plugs into the filter, with the filter staying where it is now?
If so the best solution is to leave the router by the filter/socket and use a longer LAN (RJ45) cable to the PC.
If you can't do that, and have to move the router, then a high-quality (SSTP) RJ11 ADSL cable is almost as good.
(they're not that expensive - around £5 for a 5 metre one)
Flat cables can always pick up interference and reduce your speed; flat phone extension cables are so bad you should never use them.