Backup server needs access to 2 networks

Our company has two small networks.

Network 1 runs as a domain under a Windows 2012 server, which is also the DHCP server for that network. It connects to our Mikrotik router for internet service.

Network 2 runs as an isolated network (no access to the internet or Network 1). Our Mikrotik provides firewall isolation and DHCP for Network 2.

Before we added the Windows 2012 server on Network 1, we used the Mikrotik as DHCP server for both networks, and we added a firewall rule to allow our backup server (on Network 1) access to the file server on Network 2.

However, the addition of the Windows 2012 server domain and its DHCP control over Network 1 seems to be blocking the backup server’s access to the file server on Network 2. We can still ping the file server on Network 2 from the backup server, but we can’t open up a file directory either by IP address or by server name.

Do we need to add a routing path on the Mikrotik, change the firewall filter or what? Probably an easy solution that’s staring us in the face.

Thanks in advance!