So... I don't understand how PS4s function. Bought game on wrong account, I think. Suggestions?

Hello, all. I am so sorry about bothering you guys with this. I don't do modern gaming much at all. I'm more of a PC gamer, usually. But my friends are playing Destiny 2 on the PS4 and my roommate conveniently has a console that they don't really use. My old, old, old account (the one that has a dumb username from when I was like 12) is on the PS4, along with his account. Obviously, he's the primary user and I told him to buy it in his name because I wanted to make a new account and delete my old, dumb named one. He bought it on my old account, and activated the free 14-day PS4 Plus multiplayer thing on my username. Is there a way that I can make a new username and still play online? From everything I've looked into, because dumbname69 is the holder of the game/PS4 Plus and is a secondary account with the console, my new account won't have access to it. I don't want to make dumbname69 primary cause it's not my PS4 and I'm sure that would just open up a whole new can of problems for my roommate.

Suggestions? I'm limited in my knowledge, and unsure if I'm googling the correct issues to learn how to fix this issue, if it even is one.