If you plan on using a reverse proxy, you will have to define your domain or subdomain as trusted proxy domain.
The relevant nextcloud configuration file to define this is located at:
/<your-volume-path>/config/www/nextcloud/config/config.php
Adjust the PHP file and put in your subdomains as trusted. Something like this:
<?php
$CONFIG = array (
...
'trusted_domains' =>
array (
0 => 'nextcloud.example.com',
1 => 'nextcloud.anotherdomain.com',
),
'dbtype' => 'sqlite3',
...
);