It is a good adsl modem router; but 3g dongle support is very bad

When I connect the supported 3g dongle,I get a dwnld speed of only about 1.5 Mbps through the TPLInkW8968,while when dongle is connected directly,I get speeds of over 5 Mbps.The Upload speeds is even more pathetic,the speed is only about 150Kbps