If you have a good signal from AT&T (-102dbm or higher [lower negative number]) it should be fine. (If you don't, you're on the wrong carrier, not the wrong phone. You always choose the carrier fist - by coverage [or signal strength] where you need coverage.)
It also depends where in South Florida. I have AT&T and live in the Brickell area and despite the AT&T coverage map looking well, I have horrible reception inside my apartment. Maybe it has to do with so many buildings around or something. I don't know.
As someone stated, this is more of a carrier issue most of the times than the actual phone. I've yet to see the specs for the OnePlus 7 on their site (perhaps tomorrow it will be listed). I'd compared the bands listed there to those listed on AT&T.