You may find some recommendations in these related discussions:

https://www.researchgate.net/post/Publishing-your-thesis-as-a-book-and-also-publish-journal-articles/1

https://www.researchgate.net/post/How-to-turn-a-dissertation-into-a-book/1

https://www.researchgate.net/post/How_does_a_thesis_convert_into_a_book/1

https://www.researchgate.net/post/Which_is_the_best_publisher_to_publish_the_PhD_thesis/1

https://www.researchgate.net/post/I_want_to_publish_my_thesis_as_a_book_What_authentic_publishing_houses_do_you_recommend/1

https://www.researchgate.net/post/Who_can_provide_information_on_available_free_of_charge_academic_book_publishers_where_I_can_publish_my_dissertation/1

But be warned of Lambert:

https://www.researchgate.net/post/Opinion-required-on-Lambert-Academic-Publication-is-it-okay-to-publish-thesis-through-them/1

To publish your research thesis as a book: