there is something wrong here if the diag mode rpm display is always showing "0"
did you buy the bike 2nd hand? and who fitted the add on rpm unit?
could be something wrong with the way it has been wired. Can you go back to standard by removing the unit and see what happens.
in the old days a meter could read the rpm by sniffing the spark plug HT pulse. Could your plug lead be damaged.
I would take it to a dealer for a check specially in the wake of the diag display not working as it should.
ONe last thing, assuming you are in the uk could this bike have been restricted for lower class license?
Incidentally, I was put out by the lack of rpm initially, but soon got used to the feel of the engine vibrations and note to tell what was going on and frankly now, I do not need it