My 3G cell C dongle works well when plugged directly into USB on my PC with 3mbps download speeds. However when I plug the USB 3G donlgle into a Dlink wireless router and connect via wifi to the router, my download speeds crash to 0.27mbps!! Upload speeds also affected.

How is this possible? I assume it can only be the router wifi settings. I have disabled QOS and fiddled with other settings but dont get any improvement. I am now out of my depth and would appreciate a few pointers.