Before installing the microcontroller, solder each of the four jumpers on the top side of the PCB. Don’t forget this step, as it’s very difficult to solder them after installing the microcontroller.
The jumpers are located on the top side of the PCB, underneath where the microcontroller is to be installed.
Do not solder any other jumper than the four jumpers on the top side of the PCB. Soldering a jumper on the bottom side of the PCB causes the OLED display to malfunction or even be damaged. In the event that you have accidentally soldered a jumper on the bottom side, remove any solder with a solder sucker tool, and remove any trace amounts by cutting a line between the two jumper pads affected with a hobby knife.