[i9300][CM12.1] No MMS download on WiFi, MMS works otherwise

I recently ditched my original Samsung ROM (Android 4.3) and flashed Archidroid 3.1.5 (based on CM12.1) in its place. I'm loving Lollipop 5.1 and have gotten everything working - except for MMS. When I'm away from WiFi, I can send and receive MMS no problem. When on WiFi, I can send MMS but can't download them. I get the text saying there's an item to download, but when I hit the download button it spins for a while then gives up. Needless to say, this didn't happen before I upgraded.

I've checked my APN and have verified that I have the right settings for an unlocked smartphone, according to the AT&T site. Can anyone advise me on how to fix this? I'm already tired of having to turn off WiFi every time I get an MMS.

Thanks!