How to add new mobile device to microsoft account when I have already done a factory reset on the old device

Question

I have a new mobile device - Samsung S25 - and did a factory reset on my previous mobile device a Samsung S20 - but I did not realise I needed to do something wit hthe S20 first to ensure the new phone could be added to my MS Authenticator App. I seem to be going around in circles trying to register the S25. I've already tried searching for solutions online and contact the microsoft help chat - they confirm my account is working - but say they can't help with a questions about adding a new device and recommended I post the question here. Thanks in advance for any assistance you can offer.