Phone not asking for NUC code

[SEG30]

I am going to the US for 4 months and need to unlock my phone so that I can use a US sim.

I have a Huawei P20 and have received the NUC code from Vodafone.

When I first put the US sim in the phone I was prompted for the NUC code but unfortunately entered the wrong code (only once), and now I can't get the prompt up again to enter the correct code.

I've tried swapping sims several times.

How do I get the NUC prompt again?

[AnnS]

Turn the phone off @SEG30 and follow the information from this link: Unlocking 

Insert you alternative network SIM

Turn on your device. You’ll be prompted to enter the NUC

Enter your NUC

Press OK

[SEG30]

Thanks @AnnS 

I hadn't tried turning it off previously - but it still doesn't work.

I've put the new sim in and still no prompt for the NUC.

Any other ideas please?