Ok
Lets assume that the nextcloud is install on cloud.domain.tld and you want to use same nextcloud instance for cloud1.domain.tld…cloud_n.domain.tld . Follow these steps:
Install the Letsencrypt for all the domains eg. cloud1.domain.tld,cloud2.domain.tld and so on.
Go to /var/www/nextcloud/config/config.php and add the domain you want to use to the array. eg.
), Note: localhost and cloud.domain.tld should be there before hand as the main domain on which nextcloud is installed is there on 1.
Copy the nextcloud.conf from /etc/nginx/conf.d/cloud.domain.tld.d/nextcloud.conf and paste to the domains you want to use as alternate domains for nextcloud. Eg. paste nextcloud.conf to /etc/nginx/conf.d/cloud1.domain.tld/ , /etc/nginx/conf.d/cloud2.domain.tld.d/ and so on.
Give the config.php the app permission.Eg. chown nextcloud:nextcloud /var/www/nextcloud/config/config.php
Restart ssowat and nginx service nginx reload && yunohost app ssowatconf
Hopefully all the domains should work on web and on the clients.
Note: You could use the redirect app and paste the content of nextcloud.conf to the redirect.conf so that backup and restore could work out of the box. If you add the files manually as explained above then you have to backup and restore the files manually too or repeat the entire process together except the editing of the config.php file(point 2) after restoring the nextcloud.