Going through pretty much the same thing with my Audi right now. I retrofit an RNS-E into my b5, but the radio didn't come with a code and it's from a 07 s4, so it's a mk1 RNS-E unit that doesn't show the serial number on the lock screen or anywhere on the body of the unit.
The only way I can tell, from weeks of research, is to get the serial number from a buddy with Vagcom by plugging it it, go into the advanced ID menu and it should appear. Write that number down (starts with AUZ), then call Audi of Canada, speak to a service representative, give them your VIN (doesn't have to match the radio) and the radio serial number, and thy should be able to give you the unlock code within 24 hours.
That's what I gathered if you're like me and are having difficulties with RNS-E's. hopefully my experience somewhat helps.
Audi can't do shit if you take it to a service centre without a serial number. Their system blows for stuff like that, compared to VCDS or Vag-com, trust me, my car was there for about 4 hours today and they weren't happy about not being able to charge me either.