DOS net use access denied to share

And you are sure that the old server was using NetBIOS over TCP/IP, not over NetBEUI or some other protocol?  It has been a long time since I messed with networking on a DOS machine, but not as long as some.  We also still have DOS based equipment where I work.  But when they blocked all the protocols that DOS supports on the corporate network, I had to change them to dual boot with a server upload/download performed via Windows and the machine software run in DOS.

What does NETSTAT -A  return?  Can you turn echo on during the execution of your AUTOEXEC.BAT and post a screen dump of when the networking commands are executed (obfuscating the user name and password if you need to)?

Have you determined if your CNC software will run in FreeDOS or "DOS7.1"?  Both have better networking than DOS6.22.  FreeDOS is, as the name suggests, open source and free.  "DOS7.1" is extracted from Windows 95/96/ME, so you can legally use it if you still have a license for any of those.  There are batch files around that will extract it from those install disks, but you can find locations from which to download it, too.