ATT Business Fiber and IPv6 Prefix Delegation

Earlier this week I had ATT Business fiber installed in the new apartment. This building was gutted and rebuilt in the mid-2010s, so there was already ATT UVerse fiber in the utility closet. Installation was fairly trivial; the technician showed up with a gateway (looks like a BGW210-700). Four ethernet ports on the back, one port which goes to the PON (the thing already screwed on the wall with the fiber going into it), and power.