I know T-Mobile bought Sprint, which came with their CDMA infastrcture, but I assume they are phasing all of that out to switch to GSM fulling. As long as it isn’t carrier locked, which I would find unexpected, it should work. Maybe not all of the bands supported by the phone, or all the bands supported by AT&T may work, so you may have slightly less coverage than you otherwise would, but that’s all I can fathom.

GrapheneOS probably won’t. Their main dev literally jokes about the phones security. While GrapheneOS was originally made for Pixels.