I can’t get the entire view into focus

This is what’s known as spherical aberration. You might be surprised to learn that even the Hubble Space Telescope suffers from this same problem, where there seems to be a ring of best focus. It’s something that reflector telescopes are particularly affected by.

Solution

The only real cure is to replace your telescope’s mirror – take it to a shop if you’re not comfortable doing this. If you’ve purchased a brand-new telescope and it’s suffering from spherical aberration, then you should return the instrument and ask for a refund or replacement.