If it's a PON then it's not Ethernet media. You would then be looking for an ONT SFP but those are far from ordinary SFPs. They are not just dumb devices, there is a lot going on inside them since they crammed a whole ONT into the SFP, and it communicates SFI back to the host equipment as if it would have been Ethernet.

https://hack-gpon.org/ont-wo-mac/

You would need the ISP to "adopt" your ONT into their network similar to what is observed with cable modems.

Consider that AT&T fiber device to be a literal component of their network in a way that is: adopted into the network management system, configured with appropriate settings for the particular network and network segment, maybe the device is named in a particular way for their billing/CRM solution, there may need to be e911 info configured in the device if they often do VoIP service, etc.

In short, a gpon network is not quite the same as rolling to Walmart or whatever and just grabbing a replacement cable/dsl modem.

But 3rd party devices that work with AT&T Fiber do exist. Ubiquiti just needs to get with the beat.

Ehhhhh... I'm not sure the market opportunity there is large enough for them to pursue.

They're strapped with SKU as is. Ubiquiti seems very focused on two main segments right now, which is growing enterprise switching and wifi, and their outdoor wireless gear, where (if memory serves from quarterly earnings reports) is now mostly concentrated outside the US.