You need to login the web interface and change the WAN Mode to 3G mode first. However, this modem is not specially for 3G, and may have problem with many USB modems.

You use wireless or wired connection for the speed test? You can see the speed you get on the Status page of the web interface.

I also use 8980 for home, so no need to worry about USB sharing. If you don't wan't others to access, just change the username and password. For the latter one, maybe it's not stable with WiFi