Unifi USG - way out of my depth!

Home wifi was previously provided by two Apple AirPort Extreme wifi APs which are getting pretty long in the tooth, so rather than wait for a fail and emergency replacement, I thought I'd do it ahead of time.

Purchased two Unifi AP-AC-PROs to replace, along with a Cloud Key and the Unifi Security Gateway (USG).

Upgrading firmware on the Cloud Key and the APs and getting them working was relatively straightforward and I have rock solid wifi all over the house and about 50m along the road in both directions.

My original plan was to use the USG to replace the modem Spark provided with my fibre connection. But I've quickly realised I'm massively out of my depth on this part of the plan!

I've ferreted about all over the place and can't find anything that actually gets me through my bottle neck, which is even at the adoption stage of the USG.

Has anyone done this? And if so what tips would you have for someone who has enough knowledge to be dangerous, but not enough to get it working!

Thanks

Andy