This is a complicated issue. I mean, if using their WiFi causes the neighbors to have slower internet and they don’t want you using their WiFi, then I think you should refrain from using their WiFi.
On the other hand, if you’re not taking away from their internet speed (imagine they’re out for a walk at the moment) and if you’re not costing them extra money, and if you’re not snooping on their privacy, I don’t see why it would be stealing to use their WiFi. What would you be robbing them of?
But I would ask a priest in confession just to be sure. In fact, it might be worthwhile asking multiple priests.