How do I send a text automatically when my network becomes available?

[AC Question]

how to send text automatically when network becomes available

Hi. Often I'm on the subway and write a text and hit send. I assume it will automatically send once the network becomes available. But hours later I check my phone and its still there as pending.
How do I set it so it automatically sends as soon as the network becomes available.
My galaxy used to do this. Now I am using the htc one m8 on Verizon.