H2O wireless runs off of ATT towers. Been using them for years. 90 day "pay as you go" unlike some 30 day plans. I'm still a little unclear what your after here bw. It does sound like you are wanting android, unlocked phones and ATT compatible.Cricket Wireless has "bring your own phone" sim card kits, and work off of AT&T towers. Activate it at your leisure. You can even port your number over.
May or may not have issue with the phone being locked to AT&T though, but I've never worried about that before when swapping GSM sim cards around.
https://www.cricketwireless.com/support/switch-to-cricket/bring-your-own-device-byod.html