Unable to receive MMS messages from some people

I'm unable to receive MMS messages from some people. I'm using an unlocked Moto Z Play and the stock/standard Google Messages App on Android 6 Marshmallow and just upgraded to Android 7 Nougat. Still having the problem. It tells me I have received a message, but it says "unable to download, touch to try again." I can retry over and over and nothing will download. Tried wiping the cache partition and tried a factory reset. Using APN settings provided by an AT&T rep. My hunch is that because the Moto Z Play doesn't natively support AT&T's "advanced messaging," the problem is from users who have a phone with advanced messaging turned on, and there is some "advanced messaging" code tacked onto the MMS that Android doesn't know what to do with, so it's preventing the message from being displayed because it thinks the message hasn't completely downloaded. Can someone at AT&T Tech Support test this? I've been an AT&T subscriber for a decade, but I love the Moto Z Play and it's INSANE battery life, and Verizon offers the phone on their network. I don't want to switch to Verizon and accept the bloatware (hence the purchase of an unlocked phone with stock Android), but I may have to if I can't get MMS to work correctly. I use the phone for work, and "hey sorry I didn't receive your message" isn't​ acceptable when I have to use MMS to communicate with colleagues. Please help! Has anyone else experienced this?