Bus from Nanning to Ha Long.

Hi, we are leaving from Guilin to Nanning, then from whichever city to Cat Ba Island. Is there a bus running between Nanning and Ha Long? Or do we need to go through Hanoi?
We´ve also read about going through Haiphong, but we would arrive too late. Is our best option spending the night in Hanoi and then going to Cat Ba or are there ferries running after 4pm local time?