Iphone and Straight Talk APN Issues

Hi,

So my uncle from Taiwan (I live in the United States) gave me his iPhone 4 (model MC605TA) which is currently running iOS 7.1.2 and I decided to use the AT&T Straight Talk BYOP (Bring Your Own Phone) plan. It had told me that the iPhone was compatible with the program. I followed the instructions provided in the started package for the plan - inserting the AT&T SIM card and activating the plan on the website. I also downloaded the APN app/program/whatever-they-call-it which currently can be found in Settings -> General -> Profile. I then, as instructed, turned off my wifi connection, rebooted the phone, and tried to get onto Safari. Here’s where things started to get dicey -

When I tried to use safari this notification came up:

Could not activate cellular data network

You are not subscribed to a cellular data service

But in the top left corner it has the 4 dark circles and then an empty circle (which is signal strength), TFW, and then says 3G - so I know that I’m connected. Moreover, I can call and receive calls just fine, and texting was fine. I did some digging around and stumbled upon the SIM swap method which seemed to work for everyone else... except for me. I tried about 6-7 times all to no avail.

Upon deleting the APN settings that I installed earlier, I could easily access the internet without using wifi but iMessage was acting strangely (I could still call). Even though I turned off the wifi on the iPhone, messages were not sending as texts rather they were sending as iMessages. Eventually, I re-downloaded the APN settings, but I just reverted to the original problem on top of the texts continuing to send as iMessages and if I turned off both wifi and data, they would send as texts but reach the other person about 30-40 minutes late.

I called customer support for Straight Talk but after talking to a representative for five minutes who then told me that he would be back to me after a couple minutes while he tried to figure out the problem and essentially left me stranded for about an hour and a half at which point I decided that they probably either forgot about my call or didn’t want to deal with it. So here I am. Anyone have any suggestions? Or is my case hopeless?

Bonus issue:

I have a early 2014 Macbook Air running OS X Yosemite (version 10.10.2), and when I attempted to sync iMessage with the iPhone, it sometimes would register and sometimes would not register when I sent anything as a text from the iPhone and when I sent something to my friend (who was helping me by acting as a guinea pig to figure out what was going on) as a text and they texted back, I wouldn’t always receive a reply on the Macbook but would on the phone or vice versa...

Any suggestions for this issue?