I suppose xubuntu has it's own 3G drivers?
I tried KatKiss 6.x (don't remember the exact version number) for TF300T without 3G, then installed "ppp widget 2" (or "ppp widget 3"?) from Google Play and started 3g manually from this application (widget). Android will not automatically switch from Wi-Fi internet to 3G internet (as like in stock), you must switch it manually. Some bugs discovered, for example, I could connect 3G and disconnect only once, when I want to connect 3G again, I had to reboot. device. Maybe will work on some other firmwares?