If it is a mechanical braked trailer, just do what you would do if you want to reverse a caravan (similar brakes)
1. With the unit stopped, move the car forward about one foot, AND DONT TUCH THE BRAKES.
The brake mechanism probably comes from the tow hitch. If you are moving, and you press the brakes in the car, the trailer pushes the car. Then, the towhitch on the trailer compresses and puts on the brakes on the trailer. If you try to reverse with the hitch compressed, the brakes are on in the trailer. By moving the car a small bit forward, you take the pressure off the hitch, the brakes disengege and reverse at will.
Gentle reversing will not activate the brakes
If you are not sure that this is the system fitted, put a block behind the wheel of the trailer and push the tow hitch on the trailer hard. If the hitch moves back but the trailer doesn't, you have a mechanical brake system and when reversing, just to the above.