According to our partners, they said that an internet connection is a must since there is a weekly scheduled job for the subscription validation.

we had to configure our firewall to open only on Odoo Subscription Validation URL.

see here:

The Update notification must be able to reach Odoo’s subscription validation servers. In other words, make sure that the Odoo server is able to open outgoing connections towards:

services.odoo.com on port 443 (or 80)

services.openerp.com on port 443 (or 80) for older deployments

Once you activated your database, you must keep these ports open, as the Update notification runs once a week.

https://www.odoo.com/documentation/14.0/administration/db_management/db_premise.html