3G wifi router such as from TP-Link / Aztech are using usb 3G modem to receive 3g signal from 3g base station, therefore router itself do not receive any 3G signal. Then they turn 3g signal into wifi signal for local wireless access.

Thus the omni antenna you seen on 3G router are actually WIFI ANTENNA for wifi broadcasting, not 3G antenna. 3g antenna not work on that side.

To use 3G antenna to receie 3g signal, one must connect it to actual 3G receiver device, ie:
a) 3g modem (usb/pc card etc) - then u can plug in 3g modem with attached antenna to 3g router for sharing.
or
b) 3g gateway modem router such as Huawei B660

* 3g gateway modem router is all-in-one built-in 3g modem + router + wifi (if any). the external antenna socket seen on them are for 3g antenna, not wifi instead. they use internal wifi antenna.