Thank you very much Radek,

I have wired everything and checked all connections with a multimeter. The motor just won't work. I'm starting to think it must be some sort of software issue, for instance missing libraries.
I attached pictures of the RPi and board, and screenshots of the "Astroberry Focuser" tab under INDI. Please not that the default port is /dev/ttyUSB0. This is weird, why should it be USB?

When I send move commands via Ekos, the voltage on SLEEP rises to 3.3, so I think the RPi does send commands. I checked the voltage of the motor at rest, and the readout is 1.5 volts on each of the 4 pins relative to motor power supply GND. Strange.

Vref does not go beyond 1.2V. Is that enough?

Any explanation?

First of all your stepper motor does not look like 28BYJ-48. Could you specify what motor you're using?
Second, does the stepper motor vibrate or gets warm when steps are sent from the driver? Check it out carefully. Based on your pictures I cannot verify if its connected properly. Pictures showing connectors to the stepper and stepper controller and RPi would be helpful.
You can ignore serial port in the driver. It's a leftover and it's not used. I will remove it from the code for transparency.

It's not a 28BYJ-48 (unipolar), but a Nema 17 (bipolar). I don't think the a4988 can drive unipolar motors, so I purchased this one for testing. The motor does not vibrate nor gets warm. Looks like no current is flowing at all. Maybe Vref is too low. The voltage on the motor pins on the driver board rises to 4.6 V while I'm sending commands. Starting to think it's a matter of current.

Are you sure it is a 5v stepper, and not 12v?

On the Amazon page it says "nominal voltage: 3.45 V, nominal current: 1.5A (monophase)".

Ok. But 1.5A exceeds the 1A the a4988 is designed for. In any case you would need to check the Vref pin - but pls check the pololu Instructions first...

Here is a link:

reprap.org/wiki/Pololu_stepper_driver_board

Forgot to mention, there are loads of instructions on YouTube on how to convert the 28byj48 into a bipolar stepper - it is actually fairly easy to do.

Best, Helge

I DID IT!!!! I took the risk and increased the voltage to 12 V, and not it works! I have no idea why it says 3.45V on Amazon, must be a typo!!

Fantastic, well done! - then current will presumably be lower than the 1.5A. Can you check what current is drawn?

I was also running a 5v stepper at 12v. Unless it is getting warm, should be ok - but I am not an electrician (=disclaimer).

That's what I thought after I saw your pictures
I'm glad you got it working!

Is there a way to have INDI calculate the necessary number of steps for backlash compensation? Or should I determine it by hand?