Connection to external DB between Own DBs

You should look at replication rather than connection to external DB. You can configure replication to synchronize objects from the master vault to as many other vaults as needed. Only prerequisite is that the properties in the object type on the master vault also exist in all other vaults and have the same alias.

What if I dont want to store information on a server that has access to the internet? Are there any options to access information on my internal server from the external one only upon request?

If you give us more information about what type of prolems you have on "connection to external DB" part maybe we can help on that end.

https://userguide.m-files.com/user-guide/latest/eng/Connection_to_external_database-1.html here is the guide for it

Replication between vaults work in stages:

1. The master vault creates a replication package. You decide the destination. It may very well be a folder on your local network. It could also be on protected blob storage or similar on the internet.
2. If the receiving vault does not have access the original destination you need to move the package to a place where the receiving vault can access it. You can use a USB drive if you want, or you can use a VPN or a transfer service like Wetransfer.com or any other means suitable in your case.
3. The receiving vault imports the replication package from where ever you made it available to that server.

Obviously, the easiest way to automate this process is to choose a destination to which both servers have access. That way it can run automatically in the background without additional manual or automated processes.

It sounds a little like you might want to use a "GroundLink proxy", although you'd be using it in a slightly odd way.  Normally you would install the GroundLink proxy on a machine behind a firewall, then connect an internet-facing machine through it to access local resources.  Instead you would do it the other way around.

First of all, there needs to be clarity about the network architecture and the security policies in place.

What exactly do you want to synchronize?
Perhaps it is not accidental that an architecture with vaults on two different servers was chosen.

You most likely want to replicate only metadata (nomenclatures), and not documents or other sensitive data.

The suggestions your colleagues are giving are reasonable and correct, but without clarity on the infrastructure constraints they cannot be implemented.

In your case, it may be appropriate to build a custom solution (for example, a VAF extension) on the internal server that, if it has access to the external server, can synchronize only the required data.

Do documents and/or other sensitive data need to be synchronized from the external server to the internal server?
Does the external server have access to the internal server, and if so, which ports and protocols are allowed?

It is best to contact the reseller and discuss these questions with them.

What I need is to have a duplication prevention mechanism in place. When a customer input its data on the External server to checks based on a criteria whether there is already customer(s) with the same data on the Internal server. We have strict requirements for unique values in the metadatacard for every customer.

Once the Customer passes the state of approval it must be completely removed from the external server for security reasons thus migrated to the internal. We cannot duplicate all the customers on both location.

When a new Customer signs for approval then the External server have to ask the Internal Server whether there is already Customer with the same data on the Internal server, if the answer is No then we are good to go, but if there is we need to throw an Exception and stop processing.

To duplicate the data and synchronize it between the locations is not an option to us. Is the Ground proxy the way to go forward?

What I need is to have a duplication prevention mechanism in place. When a customer input its data on the External server to checks based on a criteria whether there is already customer(s) with the same data on the Internal server. We have strict requirements for unique values in the metadatacard for every customer.

Once the Customer passes the state of approval it must be completely removed from the external server for security reasons thus migrated to the internal. We cannot duplicate all the customers on both location.

When a new Customer signs for approval then the External server have to ask the Internal Server whether there is already Customer with the same data on the Internal server, if the answer is No then we are good to go, but if there is we need to throw an Exception and stop processing.

To duplicate the data and synchronize it between the locations is not an option to us. Is the Ground proxy the way to go forward?