This thread is resolved. Here is a description of the problem and solution.
Problem: I would like to use different domains per language on WP multisite. There is an "error" saying: "This option is not yet available for Multisite installs". Is there anything I can do to make this plugin work and that I do not lose the content of the site?
Solution: In this case you need to use sunrise.php (https://wpml.org/documentation/support/multisite-support/languages-in-domains-for-wordpress-multisite-mode/) as this feature is not enabled by default and it is still an experimental feature.
This is the technical support forum for WPML - the multilingual WordPress plugin.