A modal verb is a type of auxiliary (helping) verb  that has no meaning on its own but it modifies the main verb, changes  its meaning and gives more details about actions.

Have to  is a type of auxiliary modal verb used to express certainty, necessity  or strong obligation which comes from outside, from external factors.

https://open.books4languages.com/english-a2-grammar/chapter/have-to-modals-of-obligation/