FYI I just had this same issue. However I tried using...
\DVR-SERVER.local\MEDIA\DVR\
instead and that worked. That .local thing seems to be relatively new and something that's required for me to access the webui as well. I'm using a Pi 4 server so maybe it's part of an update to that, I'm not sure. The webui should probably be updated to show this in the "Access your recordings via..." part.
Also I don't see the smba share listed in the Network section of my Windows Explorer. I have an older NAS that uses v1 shares so I already have the old shares option enabled. My only guess as to why is the work group might be wrong? I don't see an option anywhere in the webui to see, or edit, that so I'm not sure.