[Only applies to users with iOS; as for Android, I'm not too sure.]
I doubt there's anyone who had the same problem as me on here, but basically, I couldn't open the GBA4iOS 2.1, because I couldn't "verify" it, or something similar. I looked up different ways to fix the problem, and am combining them all to make one big guide.
Do not delete the app yet.
Need: Dropbox and iTunes account
Here's the long process I went through:
1. Hook your phone up to a computer (I used a Mac) and connect to iTunes → sign in → go to the phone symbol in either the top left or right corner → go down to File Sharing → scroll down to the Apps. Click on GBA4iOS and you will see files appear to the right. Ctrl+A (this would highlight everything) and click "Save to..." and create a folder, which would put everything there. Delete the GBA4iOS app.
2. You can disconnect your phone from the computer now. Now, while you're on your computer, go to Dropbox (download it, if necessary) and upload the files. Then go to your phone (or whatever iOS device you're using) and get the Dropbox app from the AppStore. Log in.
3. Now here's how to avoid the "verify" problem. Go to Safari and enter the link: iosem.us and download the icon to install (I do not believe it matters which one you choose; but for reference, I chose the one with the red border). Once you've download one, enter the app, and click on "Apps":
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Scroll down a bit until you see GBA4iOS and download the app from there.
3. Now that you've got the app and save files, remember to Trust the app in settings and then enter GBA4iOS. Scroll down until you see the Dropbox Sync option, and turn it on. The first few times, my account couldn't connect or Sync, so I had to delete the app and try again multiple times.
4. Go ahead and download the game that you have the savefiles for, OR, open the game that you uploaded from iTunes to Dropbox. To do this, click on the file:
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There will be a blank page (No Preview Available) then click on the three dots at the top right corner. Find the "Export" option → Open in → Copy to GBA4iOS.
5. Everything should work. The first few times I tried it, my save files wouldn't show up in my game, so I had to restart, logout, and move files into different folders several times.
