[N1 (Mirko)]

Eyepieces generally sit further from the objective than cameras*. Start with the focuser screwed directly to the tube. Point the telescope at a bright star, wind the focuser all the way in, put the eyepiece in then start racking out the focuser and observe the donut formed by the defocused star. If it's getting smaller, you're on the right track. If you hit the end before things are sharp, add extensions (or a diagonal). If you see no donut to begin with even after making sure it's pointed at the star, add one of the two M90 25mm extension rings, or a 2" extension before the eyepiece and restart the process as above.

If, however you are still too far out even with no extensions or diagonal in the path, and the focuser all the way in (i.e. donut gets bigger as you rack out from there), then I suspect your mirrors are not at the correct spacing from each other. That's very unlikely though, especially if the RC is new and your cameras focus fine*. But if that's the problem you'll want to get that fixed because the distance between primary and secondary is absolutely critical in RCTs. Beforeyou do anything like that, you must read up on the subject or it will become a major headache.

*More simply, you could establish roughly where the camera's senor comes to focus, and move out from there. The EP should focus a few to a few dozen mm behind that spot, depending on its design.