iPhone not receiving group texts

My other iPhone and Android version friends send/receive group texts, but I never get them. Someone just copies the content and sends me an iMessage or a SMS separately. I also never get pictures from a non-apple user. I've got (under Cellular) Wi-Fi assist off and Cellular Data on; (under Messages) iMessages on, Send as SMS on, MMS Messaging on, and Group Messaging on.

This has been happening for a couple of YEARS. I've switched from the 6 to 6s in that time, same problem. I'd rather not completely reset my phone to factory settings, though it may come down to that. But since I switched to a 6s last year, restored from a backup, and still can't receive group texts, I'm not inclined to go that route for now. Any help is welcome!

​[Re-Titled by Host]