Hi Chengfansoon.
Can you show a dir-655 setup for 3G USB modem? (e.g. Print out of display)
This router come with firmware 1.30WW.
I heard some people upgrade their firmware and after that they are unable to use the 3G modem DWM-156 on this router.
The setting is very simple
On the USB type page, select 3G modem
than
APN = (given by your mobile internet provider)
dialup number = *99#
that is all the setting you need.