Have a look at that multi-pin connector with the red black green blue yellow thin wires. It looks like in the photos that the sequence is wrong, but the photo isn't clear. Can you confirm between the two sides of the connector that green goes to green, blue to blue and yellow to yellow, It's very common that our Chinese friends who make these things get the wires mixed up.
The two blue wires with the small black connectors are probably the speed limiter. When connected, speed will be limited to probably 15mph, and disconnected unlimited. Your problem is probably coming from something else. I'd be very surprised if the bike is intended to be limited to 12mph.
I think I can see a spare connector red green and black, which will probably be for the throttle if you want one.
Rather than send the bike back, do as Frank suggests and get a new controller and throttle. Then you'll have a nice bike for very little money. As long as your battery gives about 40v when charged which you must check with a meter, you can get a controller and modify it to give much more power. I have one already modified from my old bike if you want it for not much money, but one or two connectors have been cut off, so you or I would have to solder on suitable replacements - perhaps cut off your present controller.
