The Verizon 6s is carrier unlocked, has a SIM slot, and has sufficient GSM band support to use AT&T as a network provider. That's the good news.
The bad news is that Apple has locked down iOS to the point that you cannot edit or change the MMS APN settings with AT&T providers, MVNOs included. This means that Apple dictates which providers you can and cannot successfully use with an iPhone on the AT&T network if you don't want a core mobile service to fundamentally break (
here's their blessed carrier list - there's only two AT&T providers on there that I'd actually recommend, and that's Consumer Cellular and Truphone, and Truphone is not for people needing anything more than basic PAYGO).
All phones need MMS support to handle 160+ character count SMS messages, group messages, and multimedia messages. By locking you out of MMS settings on your phone, Apple has given themselves the option to break a core and increasingly necessary mobile communications protocol between all phones if they don't approve of the carrier you pick. Naturally, iMessage still works over data with other iPhone users (given you utilize a third party service to switch your internet APN settings on your iPhone over to your AT&T MVNO of choice after every single iOS upgrade) and you have that network connection available... but good luck getting any communications sent through MMS with anyone using literally any other phone. Of course, if you can survive without MMS, go for it... but if you're looking at an "unlimited" plan anyway, you're not likely in a position to be able to swing that.
If T-Mobile coverage without roaming is a viable option for your area, the fool device would probably work reasonably well with US Mobile. Otherwise, consider a used Lumia 640 instead if you're wanting a smartyphone on an AT&T MVNO.
