I struggled with this problem and I found a simple fix. Some how the value gets changed aft a few uses. This is aimed to frustrate you. Start EKOS. Go to the INDI window click on the Connection Tab under PORT on the right hand panel change USB1 to SUB0 and click the set button.
I have the same mount and like you it is my first mount. I would like to know how it works for you and how Astroberry is working. I have a Sony A7R4 and I have a few issues. Guiding with the CEM40G uses the V4L2_CCD the iGuide selection in INDI is wrong. When your run this got to the INDI controls and the image_info tab. Change the max width to 1280 and the pixel size to 3.75 before starting PHD2.
I got rotten guiding results about 7 arctic p-p. The unguided mount works better than the guiding with PHD2. I also found that KSTARS crashes a lot. IDK Why.