Thanks a lot everybody!

Actually I bought cables a couple of days ago because clearly I need to clean & lube and put a brand new cable in.
I made this post basically to know if I had to do all that and see if it's working, of if the RD was the problem meaning that if would have to be replaced anyway.

I'm quite busy with work at the moment but i'll do everything you guys suggested in a near future and let you know how it goes!

Thanks again everybody for the help