There are two serial codes on the unit one is stuck on with sticker and other is written on unit.

The sticker one

18A00##########

And on the unit

18A00#######

Which one should I use to register?

I realise it is a long shot but just wanted to double check!

"Written on" I should have said printed on the manufacturers label the other is a sticker similar to the the wifi password sticker.

Hello @Cat_Draytek,

You can use this link to check your serial number - it should be 12 digits:-

https://www.draytek.co.uk/support/serial-checker

and you'll also find a Draytek sticker on the side of the box the router came in which should match the serial number on the unit.

John.