After many, many attempts, I finally got autoguiding to work. There were too many mistakes to mention here (chose a hot spot as a guiding start, calibration step size too small...) But finally it worked.
I tried it then by having a star (Altair) smack in the middle of the reticule - and it stayed there! Tomorrow I'll try to take pictures of M57 with eyepiece projection!
Not so good news: I had lots and lots of mount issues ("No Response 16") - will have to mail Celestron support about it.