How can we restart elementary school classroom management?

Hello! I've been working as an elementary school teacher in Korea for a few weeks now, and I feel like I made some mistakes in classroom management from the beginning. I teach students from 1st to 6th grade in Gangnam, and I thought I would have a Korean assistant teacher to help me. However, I ended up teaching alone, and I focused more on preparing teaching materials and grading tests, which caused me to lose track of the classroom atmosphere. Now, after a few weeks, some classes are not following me well, and I've been advised to be stricter. It's a bit late, but I don't know how to regain control of the classroom atmosphere. Does anyone have a similar experience? The methods I've tried haven't been effective. I would appreciate any advice!