different domains disabled in polylang

Short: In Languages - Settings - URL modifications, the options The language is set from the subdomain name in pretty permalinks and The language is set from different domains are disabled. Why is this and how can I make these boxes selectable? I am particularly interested in the last option (different domains).

Versions: Wordpress 5.5.1, Polylang 2.8.3

Background: My goal is a bilingual website (single web site install using polylang). I have two domains domain.aa and domain.bb for the languages AA and BB, respectively. Both are pointing on my document root and the .htaccess is pointing to the wordpress directory:

IfModule mod_rewrite.c
RewriteEngine on
RewriteCond %{HTTP_HOST} ^(www.)?domain.aa$
RewriteCond %{REQUEST_URI} !^/wordpress/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /wordpress/$1
RewriteCond %{HTTP_HOST} ^(www.)?domain.aa$
RewriteRule ^(/)?$ wordpress/index.php [L]

RewriteCond %{HTTP_HOST} ^(www.)?domain.bb$
RewriteCond %{REQUEST_URI} !^/wordpress/
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ /wordpress/$1
RewriteCond %{HTTP_HOST} ^(www.)?domain.bb$
RewriteRule ^(/)?$ wordpress/index.php [L]
/IfModule

Then, I installed Polylang and there's my problem.