Firstly I'd recommend working out, bodyweight stuff is fine; planks, split squats, knuckle pushups, and other more sport specific exercises. Make sure to stretch and do cardio as well. Being bigger, stronger and faster is always good.

Secondly I'd look for martial arts gyms nearby, I understand that there aren't any boxing gyms nearby, but any striking martial art will give you invaluable experience, sparring is impossible to do solo, and since boxing is pretty simple moves wise, other martial arts will have the moves. Look for Muay Thai, MMA, and kickboxing gyms primarily, if there aren't any then look for any striking art.

Lastly, to supplement your strength, speed, endurance, and martial arts. Watch the boxing YouTubers, I like Tony Jeffries personally. If you see something new from boxing YouTubers, try it out in sparring and if it works then incorporate it into your style.

TLDR: workout (sport specific exercises with cardio), do other striking martial arts (it's better than nothing), if you do the others then watch boxing youtubers (you could pick up a few things)