Accessing SMB file failes with user authentication errors

I am trying to work with a partner company to set up a file share so that I can map a drive. This has worked once and then it fails on all subsequent attempts.

We have a shared DMZ IP space and a firewall on each end, however that rules between us are wide open at the moment.

I am getting the error 'User name or password is incorrect' when try \IP. And I get '0x80004005, Unspecified' when I try \IP\share. Ping connectivity works throughout the issues with SMB.

There server is 2016 and SMB1,2,3 are all enabled. My server is 2019 and SMB1,2,3 is also all enabled.

I am at a loss and wondering if anyone has come across this before.