Unfortunately your calendar functionality is very limited.
The Check in / Check out datepicker doesn't show availability, it's just a calendar. And is the only calendar that sets the number of nights.
Beds24 staff mention that to show availability, we should use the other calendars. Property, Room and Offer Calendar.
These calendars are all static and the number of nights still needs to be adjusted on nights input of the top module Check in / Check out.
Why not implement a better UX? Like AirBnB does.
The guest should be able to see the availability in all calendars or at least be able to change the range of the stay.
Here is an open source date range picker that uses Shadow DOM, it's lightweight and has no dependencies.
https://github.com/easepick/easepick/
An example of the functionality that I'm referring:
https://easepick.com/examples/hotel.html