When a DFS Share is not mounting, this could be due to one of 2 following reasons:
1. The DFS Namespace is not running on an AD. Kernels prior to 4.11 will not be able to mount these shares on SMB versions higher than 1.0.
Solution: have the DFS Namespace installed on the AD, or install an HWE kernel
apt install --install-recommends linux-generic-hwe-16.04
2. The Fileserver the endpoint resides on only propagates as NETBIOS name and not FQDN
Solution: enable server FQDN, or add the <ip> <fqdn> <netbios name> to the /etc/hosts file