but couldn't find the data sheet for it (1855AS)
I asked around in my country and one guy told me to try attaching one side to 5v and the other to GND. Some LEDs worked fine and others didn't light up at all. In the end one if not more got burned.
So I'm here asking if anyone can tell me a way to figure out how it works.
I would appreciate the help!
As you are testing the correct functionality of LEDs on matrix board, which means the LEDs are multiplexed together. The fastest way to check the pinout is to use a multimeter in continuity test mode and then connect the positive or signal wire ( the red one ) to an fixed pin (usually start with one ) and the with negative or gnd wire ( the black one ) connect it one by one to the all the remaining pins. When you have finished move the signal wire to the next pin and rescan all the pins, repeat all the steps for every pins on the matrix board.
I don't recommend you to use directly 5v because it can be dangerous and you can damage the LEDs of the matrix display as there isn't a current limiting resistor.