I'm having the same issue with Textra. I'm not sure when it exactly started. I think when I updated Quantum ROM to 3.15, but maybe when I froze the stock messaging app. I froze messaging because I would get an alert for incoming message both from messaging and from Textra and even if I swiped the messaging alert away, it would just pop back. I've tried unfreezing messaging and that didn't help (but I'm not getting the extra alert, now). I've found the APN settings on another post in xda forums, but they weren't different from my own. I then when through all of Textra settings and found one for APN settings and it had different MMS APN settings than the phone (it was set to cingular instead of att). I've made them match now and am waiting see if I can receive MMS again. I'd be a little surprised if this fixes it, though, because Textra didn't have any trouble with MMS last month and I never went into the setting for it before.