If one doesn't visit university/college and doesn't even take any courses in theoretical physics and learns PhD-level physics from other resources without stepping foot in college or university; Would it then become possible for him to do research?
Most people I've seen emphasize that "you need a real theoretical physicist as a teacher in order to become a theoretical physicist, and that will only happen if you go to college or university" but the Internet exists for a reason.
Most theoretical physicists that I have questioned about this, just have a feeling of "I crossed the river, met a lot of people, cut many trees and spent a lot of time living in the woods coarsely to get a single apple. But he didn't do all of that and instead laid comfortably in a bed and received a single apple without doing anything." In other words, it invokes jealousy or something similar.
In my opinion, learning what a doctoral candidate in theoretical physics knows isn't impossible but can be done with some patience and hard work, without going to college. But is it even possible to formulate a thesis or do research in the field and submit it to journals?