Need help with IIS FTP Server

Question

I am attempting to setup and connect to an IIS FTP server, but can't get past error 530, win32-status 11001, sub-status 37. I've spent hours looking for an answer but have struck out. Specifically, I know that 11001 means "host not found", but have not been able to discover what setting to change (or what connection property to change) to get it to work.

The site is running on Server 2012 and is setup with the following options:

Under the server node, FTP settings:

FTP Authentication: Anonymous Authentication - Disabled, Basic Authentication - Enabled
FTP Authorization Rules: one user - TestUser2 - with Read permissions
FTP Firewall support: Datachannel port range: xx000-xx100
External IP Address: server's external IP address
FTP SSL Settings: Require SSL Connection; use 128 bit encryption

Under the Site node:
FTP Authentication: same as above
FTP Authorization Rules: Contains two users enter on this node [Administrator (read/write) and TestUser (Read)] but it also shows the user entered above.
Bindings: Type - ftp, IP address - server external address, Host Name: xxxxxx.xxx, port - custom port number (i was using port 21 but the log files show many-many connection attempts from all over the world; so using a custom port number to try and minimize those junk hits)
FTP SSL settings: same as above

I am attempting to connect using FtpWebRequest under VB DotNet 4.7.8.

dim oRequest as FtpWebRequest = FtpWebRequest.Create(ftp://xxxxxx.xxx:{customPort})

oRequest.Method = WebRequestmethods.Ftp.ListDirectoryDetails
oRequest.Credentials = new NetworkCredential(--see below--, user-password)
oRequest.UsePassive = True
oRequest.UseBinay = True
oRequest.EnableSSL = True

Then try to use the GetResponse method.

(This code has been working for years using a vendor supplied ftp site, but when they were bought out and raised their rates by almost 200% we decided to move our FTP site to one of our existing web servers. Ugh... more work than expected but hopefully it will pay off in the long run.)

For the user, I have tried, based on various posts:
TestUser (or TestUser2)
websitename:TestUser
internal-server-name:testUser
.\TestUser

and they all fail with the same 530 error. When I inspect the IIS log file, it show an sc-status of 530, sc-win32-status 11001 and sc-substatus of 37.

Since it is creating an IIS FTP log file entry, I know that the server/port combination is ok and that it is using SSL as expected.

Now, I've exhausted my list of possible setup variations and I'm at my wits end. (I can't find any listing that defines substatus 37.) As mentioned above, I just can't figure out what to change, either on the server or in my FtpWebRequest to get this working.

Thanks in advance...