The connection error can be caused by several things:
1. Is your database file open in the trial FMP 12 on your PC or Mac?
2. Did you turn on FIleMaker Network (File>Sharing>FileMaker Network…) and set the network access to "All Users" ?
3. Do you have a static IP Address assigned by your isp? If not the IP Address may have changed especially if you have restarted.
4. DId you change the port forwarding setting on your router from 5003? (This should be OK if you had it working before.)
If you are the only user, you can copy your database to the FM Go app on your iPad using iTunes; use it locally on the iPad; then copy it back to your computer later replacing the original file. Before making these copies, quit the FMP 12 on your PC or Mac so that there is no conflict with open files.