How to unlock a wifi iPad locked by Find My iPhone?

I lost my iPad a couple weeks ago, and it turned up at a shopping mall close to home and I was notified via Find My iPhone. I immediately locked it with Find My iPhone, and subsequently retrieved my iPhone. Unfortunately, since I have not used the iPad for several months, I forgot the PIN of it, and therefore there seems to be no way to unlock my locked iPhone .

I tried taking the iPad to a open wifi location, to see if Find My iPhone can see it again, but it doesn't want to join any open wifi. I also tried connecting it to iTunes, restoring it to factory default setting, restoring it to latest iOS, and various other suggestions I found on the web. None of it helped.

Any suggestion from the community would be much appreciated!