As stated by @dexdexdex , Remote WebUI instances are still supported.
In order to understand the root cause of the described issue, it would help if on the computer where WebUI is to be installed you could manually run one of the two commands below (the one that applies to your case among sqlAuth and winAuth), and check the result, and also the content of C:\Users\Default\AppData\Local\BigFix\BESAdminDebugOut.txt:
BESAdmin.exe /testsqlserverlogin /server=“sqlserverhost,1433” /username=“sa” /password=“password” /sqlAuth /hideUI
BESAdmin.exe /testsqlserverlogin /server=“sqlserverhost,1433” /username=“DOMAIN\user” /password=“password” /winAuth /hideUI
Note: WebUI installation runs BESAdmin as SYSTEM, so the optimum would be to manually run the command from a SYSTEM command prompt.