At least in the United States, most controllers would have no idea if you were Mode-S equipped or not, so that does not surprise me. The way to know if your transponder is working correctly is to have it tested by an avionics shop. In the USA, you legally must have the transponder tested before operating it.
We are not experts at what ATC can see in other countries, so you'll need to check for yourself if ATC can tell Mode-S and Mode-C targets apart.
There is no way the RF cable could make the transponder work in Mode-A and Mode-C modes but not Mode-S.