In Laravel Forge you have the option to check a server to allow another server in same internal network to connect.

Text at Laravel Forge states this before the checkbox with server in internal network you can connect to:

Below is a list of all of the other servers this server may access. By granting this server permission to connect to other servers, you may allow the server to connect to remote database, queue, or cache servers within your network.

Once checked the UFW firewall is update to allow the checked server access via internal ip address like 10.0.0.3. Former would get access to server you are working on like 10.0.0.2.

Currently you can add an internal network at Hetzner but access between servers will fail as you need to manually update UFW firewall

Second issue was that the database users added to database server only allowed access via 127.0.0.1 so even with firewall allowing webserver in the database users were still refused because all but ploi and root are not allowing other host.

Solution is to have checkbox to allow other server in via internal network ip, have the firewall updated and also have database users updated with host.