My phone is a Galaxy S7 and I use the default Messages app to send and receive texts. Just last night (8/20) I updated to Android 8.0 Oreo. Ever since I did the update, I haven't been able to send or receive group texts, picture messages, and general MMS. I checked my APN settings and made sure everything is still set up to Mint's specifications, but still have the issue.
Before updating to 8.0/Oreo, I could send and receive MMS without turning on Mobile Data. I assumed since it says "Access the internet through your mobile network" under the Mobile Data section that I could turn it on if I wasn't connected to a Wifi network and off when I was. And that this section was -specifically- for connecting to the internet when there wasn't Wifi available.
I attached an image of my Data Usage settings along with an error for a pending MMS message to be sent, maybe that will be helpful.