How to climb and 1 v 9 carry?

Hi everyone!! Can anyone tell me how to climb in platinum elo? I'm currently platinum 4 and i don't think i'm really climbing the latter. I only get 18LP max and lose like 20 LP max. I sometimes have winning streaks of 3 games and after I just insta lose 3 in a row. So i end up losing 9LP. Any tips on champion? Gameplay? (i'm an adc main) I have problems with playing from a behind scenario, i feel i cannot do anything to help the team because i don't have items to do dmg. And when you are behind the teams really just don't care about their adc because they don't protect me from a fed rengar ulti or something. How can I solve this? I really need some advice!! Thanks :)
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