Galaxy S4 Lollipop will NOT Send text messages

99% of the time my Galaxy S4 will NOT send text messages. I have tried the stock message app and Hangout. I sent texts and the program reports message not sent.

I spend hours on the phone with ATT support. I even swapped out the sim card.

NOTHING works!  I can get incoming texts, but 99% of the time I cannot send.

This ONLY started happening after the Lollipop upgrade.

This happens everywhere, So it is not just the towers in my area.

On that rare 1% of the time, messages will go through, but after a short time they will stop again.

I am NOT buying a new phone, my phone works great. If it definately and AT&T problem.

The ATT rep even remotely connected to my phone and said it is set up correctly.

I tried to run the ATT messenger app and it will NOT configure.

HOW CAN I FIX THIS!

I pay $90 a month for your service and I cannot fully use what I pay for!!!!!!