Samsung Galaxy Note 4 — Can't get my APN settings right

I'm a fairly basic user, a Lehman when it comes to techy stuff I'd say. I've been on straight talk gladly for 3-4 years, I now have a Note 4 and just can't seem to get my APN settings right. I've been able to send pictures and receive picture and video at times. BUT most days I get a box that says it's attempting to download an MMS message (which it never I'd able to) and I've not been receiving long texts or able to send them. When I play around it actually let's me download them when my wifi is off.