Cant consistently stay in Platinum due to teamates

I know what you're thinking, another person who blames their teamates for not being able to rank up when they're actually the problem, but seriously it's not my fault. I get into Plat 3, lose 3 games either due to Overextending Tanks or Bad Dps who go 5 and 12 (for example), derank, the cycle repeats. I play Loki and i'm not doing anything inherintly bad, i get ok healing with roughly 8k and around 15 kills a game. I don't have anyone to help me Rank up so i'm forced to solo queue and have questionable teamates. I finally started to enjoy the game but Ranked has been unrewarding and Quick match doesn't give me anything to work towards, i don't know what to do.