Welcome to Vodafone Community
First ensure the alternative network sim card your using is activated i.e. connects to network mobile data.
There should have been instructions on the email along with the Nuc.
These links may help you >
If they do not then I'd suggest to first perform a Google Search and speak with customer service on 191 if necessary to obtain the Nuc email address.
Live Chat can help too.
I wish you all the best with this situation.
Current Phone > Samsung Note 10+ 5G _256Gb > Model: SM-N976B.
Samsung One Ui 2.1 / Android 10.
Samsung Gear s3 Frontier Watch.
Samsung Galaxy Buds.