The phrase "monotonous routine" means doing the same thing repeatedly without much variation or excitement. It suggests that one's daily activities lack interest or are dull and predictable. Full definition
Between the constant diaper changes, multiple runs to and from school each day and continuous loops of Itsy Bitsy Spider your kids will be singing each and every single morning, you'll feel like you're trapped in the same monotonous routine day in and day out. (thespruce.com)