I found a way to update the AT&T fiber database. You keep calling AT&T until you get an AT&T sales rep who'll fight for you. You order DSL service with them because fiber at that address isn't in their database. (They will put in DSL service to any address, while they only put in fiber service to addrs where the database claims they support fiber; Catch-22.) The sales rep asks internally to upgrade the order to a fiber install, by doing an "Address Verification" using an AT&T fiber installation contractor. Once the fiber tech actually goes there and physically confirms that the address is OK for fiber, they update the database. You can tell if the database is updated because the Sonic website will stop denying you. You may have to actually get the AT&T fiber service installed (I did) but then you can cancel it within 14 days; it might cost you $100 or so if you're unlucky, but no more. Then you can order Sonic service there. It worked for me!
I would've been happy to do it the straightforward way, but AT&T was relentless in making every other path impossible.