iPhone 6s not Delivering Text to Androids

Hi,

I have a new iPhone 6s on AT&T - Androids on AT&T will receive my texts - but Androids on other networks usually are not. It's almost as If I text and it just goes into outer space. Any advice on how to fix this or solutions?.....

I have done just about everything from turning my LTE on and off, restarting my iPhone, turning iMessage on & off, doing a restore via iTunes.

I even when to the AT&T store and texted all the Androids in the store, and they were receiving my texts as we tested the issue.

Thanks