the Dynamo is an odd weapon. when others are using it, it seems like it's the most powerful weapon in the game. but when it's in your own hands... the tables turn, don't they?
you could be dying because you're focusing so much on getting kills, rushing into fights without really planning them through. the Dynamo is a huge risk-versus-reward weapon and you need to consider your battles VERY carefully. you are a sitting duck while swinging, or even really most of the time, as you have very little room to move.
instead, think of the Dynamo sort of like a charger. take high ground, keep a good eye on your surroundings and slam down on anyone trying to break into your territory. you want to defend, rather than offend. i know it's super tempting to want to get all the kills and seem like the best player, but in ranked, often that's not going to cut it. you need to protect the objective, claim zones, push the tower, carry the rainmaker, etc. you need to protect other players and you need to know when to get out of the way.
the Dynamo has huge ink capabilities, it can ink huge areas of ground in no time at all. now, think of Splat Zones, the zone is just one flat stretch of ground, isn't it? you can easily take a zone, and defend it. you don't even need to kill other players, dynamos are pretty intimidating to most people just to avoid you.
in short, a good player is measured by more than just their kill/death ratio. you can kill the entire team several times over but if you didn't take the zone, it'll mean nothing. take your time and don't be afraid to play the defensive if you're losing the objective. good luck!