Whether you've merged accounts or not, the little "wifi" logo should go green whenever you are online.
Verify that you're logged into iOS game center. Playing normally or merging accounts both rely on a working Game Center login.
The Game Center app has been gone for a long while now, so what you need to do is open the Settings app, and scroll down about a third of the way.
Just above Twitter and Facebook should be Game Center.
Make sure you are signed in with your Apple ID. If if shows that you already are signed in, try tapping on your Apple ID, sign out, then log back in again.
After that, restart Pathfinder. You should get a "Successfully logged in" popup, the icon should go green, and merging and general play should be back to normal. Good luck!
Just in case anyone else is having the "processing..." problem while linking android to pc, the solution in my case was similar to this solution for iOS but then logging in to the Google Play app.