Is this normal for Internet connection? please help ! need some help with speed?

so im with bigpond adsl 2+ 200gb a month and im promised 20mbps download? but max download speed ive ever seen is is 500kbps MAX!, and ive done a couple speed tests online says my net is 2.1 mbps? is this normal am i missing somthing or should i call my isp? im not sure atm what to do.