apn - enhanced phone vs. nxtgenphone

approx two weeks ago my samsung android galaxy SM-A326U smartphone with service through AT&T went berserk and for approx 40 hours i could do nothing but text.  no data, no phone.  i am NOT a techie type of person so i was completely baffled. I could not even dialo 611 to ask for support.  i did notice however that at the top left of the screen where it always indicated both the current time followed by AT&T, it still reflected the current time but instead of "AT&T" is showed "AT&T ...2". I was not able to scroll over that area or somehow make it show what was meant by the ",,,2".  Well, after the approx 40 hours of not fully working i woke up touse the restroom at 4am in the morning and looked at my phone and it must have been the exact time that my phone came back to what to me was "normal life." It shut itself down, restarted, and when it did I saw the "AT&T ...2: had reverted to the "AT&T". I watched the whole process of shut down, restart, revert. So i picked up the phone and made a call, accessed the web, and was just kinda shocked.

Anyway, later in the morning as i just kind of perused the phones settings, etc to see if i noticed anything else that was different, I was shocked to find that I did.  (Shocked because as I said, I am not a techie at all, so most of the settings are just words to me that I NEVER alter or questions cuz i dont want to screw things up. Which i have before and vow not to do again.) I noticed that the APN had changed from NXTGENPHONE to ENHANCED PHONE. So today i did a little research and read some info in some of the chatrooms and forums and tried to ADD the NXTGENPHONE APN back onto the phone.  I followed precisely but my phone would not accept to ADD another APN.  so , i ask to any of you reading this who might assist: 1) why did it just switch on me and shut my phone down; 2) why did it take 40 hours and then miraculously resurrect using a new APN; 3) which APN should i be using (and why); 4)why cant I add an APN since there is an "ADD" button; 5) what are the correct settings for both the aforementioned APNs.  And most of all, I truly thank you in advance for any assistance you might provide. I am grateful.