For first time setup, maybe try during the daytime .... say on a distant tree. That way you can work out roughly how "far back" your camera needs to be. Then tweak the focus at night time on the moon or stars.
It's possible that you just need a spacer to extend your focuser travel so that you can bring images to focus on the camera sensor. Something like one of these:
https://www.bintel.com.au/product/bi...-tubes-2-inch/
Good luck!
Jeff