Hi Glenn,
A few things;
Serato DJ Intro doesn’t require an internet connection. This software should work out of the box (pending correct set-up / driver installation) with your MC6000MK2.
Serato DJ does require an internet connection (to activate it for the first time), and you may have received a Serato DJ Upgrade voucher with your MC6000MK2 to get access to the full software.
The internet connection dropping when you launch the software is rather odd. If you are using Serato DJ Intro, does it make a difference if you remove the internet dongle before connecting the MC6000MK2 and launching Intro? There may be a strange USB conflict of sorts.
Feel free to get in touch with Serato Support;
https://support.serato.com/hc/en-us