Hi Alex!
Sorry for the late reply! I’m glad you managed to find the answer to your problem!
About the question of the function of the jumper connectors, they are used to control the step size of stepper drivers.
As you can see in the PCB Schematic, each driver (marked as U1, U2, … U8) has 3 pins called MS1, MS2, MS3 (MS comes from Micro Stepping). These pins can have 2 states: LOW (0V) or HIGH (5V). I used jumper connectors to connect MS1, MS2 and MS3 to 5V and configure the microstepping as 1/16. Other configurations can be made by using different states in MS pins as shown in the “Step (and microstep) size” section of Pololu A4988 Driver product page.
I hope I have cleared up the doubt!