Generate all urls with https

This sounds like a bug in the plugin, and they should fix it. But if they don’t, you can always add a filter to home_url and change all http:// to https:// like so:

add_filter( 'home_url', 'wpse_259675_home_url', 1000, 4 );
function wpse_259675_home_url( $url, $path, $orig_scheme, $blog_id ) {
  return str_replace( 'http://', 'https://', $url );
}