I have a 2016 SharePoint Farm hosted OnPrem with several hundred contractors offsite on public secured networks. When they try to connect to our SharePoint site, they are prompted to log in. Every time they try, no matter how they format their user name, it bounces back without ever failing out.
Loopback check is irrelevant since it's off the local box, but it is disabled, regardless.
The farm was configured for NTLM only, but I set it up to be negotiate:Kerberos, and enabled NTLMv2 since I'm assuming it's falling back to that as well as only leaving TLS 1.2 enabled, but they still can't get to it.
I've validated that every other location except this particular network operates fine.
We don't have the luxury of running fiddler from their client boxes and our insight into that network is very limited, we've requested that they help us get connected, but if we can circumvent the network administrator's attention it would be best.
How can I get these users to authenticate?