Hi VLud, given your advise I was able to find another 508 with Unlocked BSI.
1. I put the Citroen VIN on the unlocked BSI - it was accepted
2. I connected my NAC to the 508 with already changed VIN to the donor car (it began to beep-beep)
3. Changed the VIN of the NAC with the one on the donor card - now both BSI and NAC are with the same VIN, the one of the donor car (beep-beep disappeared).
4. I connected the NAC to my phone hotspot and tried to refresh the Connected Services Subscription in the NAC menu -
5. No Connected services came out.
6. Tried to put the donor car calibration (or at least one of the C3 Aircross) - it didn't help
Now what I think could be that the serial number of the NAC is in fact not matching the donor car VIN for some reason.
Now when I go back, what I can remember is that I forgot to connect the GPS antenna to the NAC when I was refreshing the services subscription, could it be the problem?
I am sure that the Connected Services subscription is Active (this could be seen in my previous post, there is a screenshot).