My understanding is that OP has chosen to use IDNet for broadband and their process is to request the serial number of the ONT. That’s just the way they roll. It’s been that way for ages.
And the solution is to contact IDNet and explain the situation. They can raise an order manually if necessary. The serial number check is just to prevent accidental slamming.
Send them a photo of the contents of the ONT enclosure and its contents. It might be either:
1. There's a working ONT, but the serial number is missing or obscured
2. There's a faulty ONT, or an ONT which has never been registered
3. The enclosure is empty
Either way, IDNet will be able to resolve this, sending out an OR engineer if required.