AT&T phones work on AIO, but you may need to change the APN settings. I think you should visit their store. I've had AIO Wireless 2 months now. It's as good as AT&T reception wise. You do have limitations on DATA 4-8 Mbps download depending whether you have a 4G or 4G LTE phone. As for them shutting it down, I'll worry about that when it happens. Right now, I'm just saving me some bucks on my cell bill and getting terrific coverage. Anyways, as for your original question, if it doesn't pick it up automatically, here's the settings that work for on My ATIV S for both MMS and Internet:
Under the internet APN, in the APN section type "ndo" and leave everything else blank.
For MMS APN:
APN= "ndo"
User Name: leave this blank
Password: leave this blank
WAP gateway (URL): "proxy.aiowireless.net"
WAP gateway port: "80"
MMSC (URL): "
http://mmsc.aiowireless.net/"
MMSC PORT: "80"