Markus chair always reclines Help Plz :)

So as I said in the title, I've had a Marcus chair for a bit now, and recently I've found that the left handle will not make the chair stick to the reclined position I want. (Which is generally to just sit straight) The chair is basically always fully reclined whenever I sit on it and it's horrible.
As we spend a lot of time home right now (yey rona... /s) I've started to get headaches because of my bad sitting position since my back, head and neck are never resting with my usually just sitting straight up. Are there any fixes to this or do I just accept my chair is graveyard worthy?

I have actually read somewhere that there is a 10 year warranty for these kind of chairs, but with me moving a couple times I have no clue where the actual tickets are or if I even have those anymore..

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