Advice for winning?

Me- Trying to get into ranked more, but I get discouraged a lot because the ELO I am in has awful game management - and I can't learn for myself because there is nobody in a high elo to show me. I main Rengar Top - yes, it's still viable (not really) - and I win lane pretty much 100% of the time. My Rengar mechanics are very practiced, and I can play the champion well. But this is not the issue. My problem is that I hit a wall every time the game progresses past the beginning of midgame. At this point, people start roaming, and I start waffling. Either I stay in lane and push and cs and kill my opponent, or maybe I go to other lanes and kill them. Both work. However, after that, when teamfighting is being tested, I fail. Once the team starts roaming around the whole map, I get lost. What do I do? DO I go for kills? Do I oneshot the ADC and try and survive? Should I just act like a fighter in the teamfight? Maybe I should try and apply map pressure - if so, then where? This is the point where I lose my advantage, and I would really appreciate it if I could get some players with experience to give me advice to get out of this hole I dug myself. Thank you all!