Server 2012 R2 + Webdav + issues mapping drive

Server 2012 R2 with Webdav setup, server is remote to office.  Attempting to connect over the internet via HTTPS.

I can access server via web browser (https://server:port/folder) - logon with defined user credentials.

I am unable to map a drive or add as network location, which is what is really needed.

Am seeing the connection attempt in IIS logs, and the following errors:

401, 5, OPTIONS, /folder, -,
200, 0, OPTIONS, /folder, -,
401, 5, PROPFIND, /folder, -,
405, 0, PROPFIND, /folder, -,

SSL is required.  Authentication is set to Basic.  Directory browsing is enabled.

Any idea why I cannot map a drive or add as network location?

Thanks