genuinely how to i get out of low elo

im a high plat to low diamond ranker consistently in each season and i honestly dont play that much im just a really solid support main and i get fairly decent matches with the standard roll here and there but stay consistently in the same place ranks

i wanted to play with my bf more so i decided to play matches on his account the past week to try to get him out of hardstuck silver. and holy shit. its impossible to climb because his "elo" is in the actual ground. on his own he lost 6 rounds in a row. the next day i tried to win a few. i genuinely did the best i could but half of seven games were purely unwinnable. im not talking about just weak playing but i mean seriously no shot matches. back to back i had: the tank ragequit after first round, a tilted dps throw bc the enemy was better than him, the groot in another game lock us in spawn with his shield so we had to surrender bc he was trolling.

i won two out of seven as a result of these back to back horrid matchmaking thats completely out of my control? i assume this is bc he doesnt play rank much but how the actual fuck is this fair? i thought it would at least help that a high stat support would affect his winrate but im at a loss. i report and go next but how do you stop getting these god awful matches???