group text messages and visual vm with straight talk

I just switched to byop straight talk (on the AT&T network) using my new iphone 6s running 9.1 and can't receive group text messages, and visual voicemail isn't working either. What's strange is that I can send group messages, and I get some replies to group texts from other iPhone users, though not all of them, and nothing from non-iphones. Searching the web it looks like this may be an issue with the APN settings, but with iOS 9 you can no longer manually edit the APN in Settings - they are locked. I've tried turning messages on and off, rebooting and resetting network settings. Nothing seems to work. My only other option seems to be swapping SIM cards. Any suggestions on how to resolve?