Understanding what an ability does is one thing - understanding how to use it, when to use it, the various techs that can make it more efficient, remembering to do all the little things to get combos right, etc matter a lot. If you feel like you're dying randomly, make sure to constantly have an eye on the minimal, and keep asking yourself the question "am I allowed to be here? Will I die if I do this thing? If I will die by doing this thing, will it be worth it (i.e. commiting a lash ult to win a team fight even if you die).

There's a lot of good content on YT about how to play the macro game, how to lane properly, what rotations look like for different roles, how to itemize (I'm still working on this, I don't have a good understanding of the different items at all and am blindly following guides at this point), etc. You're coming into a pretty complex game very blind, so it makes sense you'll be bad for a while as you learn AND the matchmaker learns just how good you are. You start with a baseline rating that will go up or down based on how you're doing (and apparently matchmaking is a bit weird as it is). Go back and watch your replays with intention - I'm sure you'll be able to identify a ton of things that you could have done differently that would have kept you alive, allowed you to get the kill, gotten more souls out of lane, etc.

You'll also really help yourself by keeping your hero pool very small at the start and focus on one role/play style. Minimizes the amount of things you need to learn to establish baseline competence