Calendar apps with Caldav support & no ads

I'm an iOS 18.2 user, purelymail user, and Caldav user. For some stupid reason iOS just doesn't support Caldav for me. There doesn't seem to be any apps that do this without integrating ads. Is anyone here using something they recommend or know how to get the native calendar to support Caldav? I try to add a calendar but my mail account isn't recognized, the only things it can add are Notes and Mail.