8-digit password prompt only when SIM is in device (it's not the PUK)
Since the PUK does not work (which usually would only be asked if the PIN has been entered wrong three times) and the prompt says "Network locked" this is most likely a SIM-Lock of the phone by the carrier.

Using a SIM from the carrier the phone is branded with you should not get this prompt and it should simply work, to use a SIM card from another carrier you will need the unlock code.

You can get this code from the carrier, most carriers charge only a very small amount or nothing at all after some period of time.