Some of the early version F@ST3864 modems were single band 802.11n only, if yours does have AC NBN, V3 AC or V3HP after the 3864, it is a single Wi-Fi band modem. From a PC which is within 10 metres of the modem, can you look at your Adaptor Properties.
Windows - Control Panel, Network& Sharing Center, Change Wi-Fi Adapter Settings, select Connections - that will show you the transmission speed of your PC to the modem - if that is showing, for example a figure between 13 Mbps to 130 Mbps, the problem may be associated with your network connection - are you on ADSL or the NBN?
Mac - click the Wi-Fi Icon to see your transmission Connection speed to the modem.
Move around the room and do a few checks on your transmission speed to the modem. Once you have established a high rate, then do a speedtest.net test for Download and Upload. If your network speedtest results are still miserable, suggest you report the problem to Optus for resolution - they may supply you with a replacement modem or fix a problem in their network or pass it on to the NBN depending on their diagnosis.
More modern F@ST 802.11ac modems (2.4Ghz and the faster 5Ghz band) are also available on Gumtree for under $50, sometimes new, unused or only used for a few months including the current F@ST5366 model, so if it was a network problem that is resolved, you may get improved in home connection speeds with a newer model modem.
