Animation montage doesn't play properly in different poses like crouch and ads, should I use additive or IK system to solve this issue or use them together ( Additive and Ik )?

Unfortunately because I am a new user in this forum, the website doesn’t let me to upload videos and pictures, however you can view the video and pics in this link below in reddit.

My issue is I can’t play montages in crouch and ads poses, left hand won’t follow right hand in crouch or ads pose, I am not sure what to learn in order to fix this problem, I hope someone guide me in right direction