Why can I not send mms & pictures with non stock messaging apps?

[Android Central Question]

I have an unlocked LG V10 H900 (the AT&T version) running Marshmallow 6.0. I am using it on T-Mobile network. I hate the stock AT&T messaging app mainly because it is always prompting to backup and sync. I have tried several 3rd party sms / mms apps but none have been able to send an mms or picture. It always times out. Standard sms there is no problem. The other thing I noticed is that the mms or picture that I am trying to send shows up in the stock messaging app even though it isn't set as default. I have the new app set as default and all apn settings are correct seeing as I have no problem sending with the stock app. I just don't get it. I do have a settings database editor app installed that I used to successfully enable tethering without root, because there is no root for my phone on Marshmallow yet and doesn't look like there will be one. Could there be a setting there I could change to help? I would like to use Android messages app but would be satisfied with anything without ads and is not AT&T branded.