Hello. What phone models are currently working with AT&T 5G network that are dual SIM and that can be purchased factory unlocked online via Amazon. The phones provided by AT&T that are dual sim in the US seem to be using the eSIM feature and can't be unlocked for use with a second carrier unless paid off in full and after a minimum stay of 3 months with AT&T. I used to have an unlocked Caterpillar s41 working on a dual sim with 2* 3G sim cards from AT&T and other overseas carrier. Device stopped being supported by AT&T anticipating the 3G network shut down.
Be careful when buying on Amazon though — there are both an international and the US models of the Zenfone 8 and only the US one (ASUS_I006D) will work on AT&T. Amazon sells a lot of international models as well (haven't checked Asus, but for other brands for sure).