FreeTDS is a pain to get working on Windows OS. Does the PHPInfo() output show a MSSQL section? If not, then there's something wrong with the FreeTDS setup. Where have you got your freetds.conf file? In your root? If not, then have you set the correct environment variables to tell Windows where to find it?
Alternatively, have you considered using the Microsoft SQL Server Driver for PHP instead of FreeTDS? It's much easier! And "safer" - Microsoft provides pre-build .dll's of their SQLSRV driver on a fairly regular basis, whereas you have to google really hard to find an up-to-date FreeTDS .dll - or build it yourself. And that's a massive pain if you aren't already a C++ developer.
See https://docs.moodle.org/31/en/Using_the_Microsoft_SQL_Server_Driver_for_PHP