Generally speaking you can connect your DM unit to the Internet in 2 different ways.
One is wireless, by connecting your DM unit to your mobile phone with a Hotspot/Mobile Router Wi-Fi mode enabled.
The other one is to plug in a GSM modem(with a SIM card) to one of your USB sockets.
The main, and the only one in my opinion, advantage of a CAR-NET is the access to live road traffic data via VW SAT-NAV and maps.
By default CAR-NET is only available to the owners with a factory fitted DM.
Fortunately the CAR-NET has been cracked and now equivalent of a VW service can be purchased elsewhere at reasonable price.
And this service works regardless of the DM being factory installed or retrofitted.