How can i reset all settings to default?

I am curious, what DNS servers are the DHCP clients using if the Resara DNS is not working? I assume the gateway? Anyways, You will need to add the DNS service to the 2003 server and create the zone. Then update DHCP scope to use the new DNS server in order for clients to properly resolve. Once you create the _msdcs zone you will need to reboot the server or restart the NETLOGON service in order for this to populate. Here are a few link that might help:

https://servergurunow.wordpress.com/2017/09/29/recreate-the-_msdcs-dns-zone/

https://www.hivelocity.net/kb/dns-configuration-everything-you-need-to-know/

If this does not populate automatically, you must ensure that the new DNS server zone IS active directory integrated and the primary server for the domain and that the new and old DC is pointing to it the this IP. Population might not happen immediately. But you can manually force replication using repladmin commands on the 2003 server if you wish, or just wait for organic population assuming everything is configured correctly.

Sorry for being off topic. Can you tell me how to demote Resara's role to an ordinary machine in the domain? Is this possible?
We need shared folders that are located on it.

This is not possible with resara. Once a DC is decommissioned it should not remain on the domain. What will happen is your domain will become "split brain"

RazDC can do file sharing it's just not in the interface yet, I will work on this.