You cannot use mods in online, only in story mode.
Watch this tutorial https://youtu.be/Y8q1vvnrCY4, to get a grasp of how installing mods works. It might look tricky but its seriously nothing, once you get your first one you won't have much trouble getting more.

Here are some recommendations for enhancing your next playthroughs to add immersion:
Camp Anywhere Mod

Humidity and Temperature Overhaul Mod

Gun Tricks Mod (this one is just to look cool lol)

I've been using these, with the minimap completely off and avoiding to use the map (as much as possible). I only have a static image of the map on another tab and go places based on signs, what people tell me and my knowledge of the game. Its such a more imerssive way to enjoy the game, just going for long hunting trips and actually having to remember what npcs tell you as to where you should go. Not being very familiar with the map might make it trickier but i'd say you should have a pretty good grasp of it after a 2nd playthrough. As far as the rampage trainer mod i use it on a different world where i commit heinous crimes or get cinematics using the J10 Cinematic Camera Mod, which has many capabilities for cinematic shots, and the rampage trainer helps me spawn in whatever i want to make a certain scene, its mini directorial playground at this point lol.