New seller (1 week old) – seeking advice on why we aren’t eligible for the Featured Offer

BACKGROUND
We just started our seller account in the UK and uploaded 4 listings. This is for our own brand and we are brand registered. We’ll be doing our own fulfillment with our existing 3PL in the UK, which has a service level that complies with seller-fulfilled prime.
QUESTION
Our listings are live on Amazon.co.uk but they don’t show up with a Featured Offer / buy button. Does anyone know how we can qualify for the buy box on our own listings? We’ve gone back and forth with Amazon support several times and only get a canned response. I’ve read other similar questions here on Reddit and some answers reference a 90-day waiting period for new sellers, whereas others report immediate eligibility.