Dick Chown Award 2016
No special keying is required to enable the code entry on an RNS D. You just enter the code using the lower right knob. In the IPs case it sounds like an invalid code was entered and the unit is locked.The information you need is likely in the manual. This is from @spike regarding a Symphony unit, your rns may be the same though I don't have a manual...
"I had a Symphony1 in my 2001 TDi and it needed the code after battery disconnection.
TP and RDS were used on my radio but later ones used RDS and SCAN to get '1000' to appear."
Many manuals don’t tell you how to enter the code (I think they expect you to take it to them)