Even more important than getting the knots out is not getting the knots in the first place. Move more all day long. Loosen up and stretch at the first sign of a “knot” forming. Take your breaks and move/stretch more than just sit and eat/do nothing. Etc., etc., etc.
When you get the knots, loosen them gently. Being aggressive can work, but it takes time and practice to learn how to aggressively remedy tight muscles and connective tissue adhesions. I would recommend seeing a Physical Therapist, Personal Trainer, your Dr., and get some clearance and advice locally.