That's an old and slow modem. Seems to only do 'g' speed wifi. There have been three updates in the technology since.
2.4 GHz, 5 GHz
You can get up to 20 times the throughput now. You can't upgrade the unit you have, only replace it. You sound like a long term Optus customer? You should be able to just call or use LIVECHAT or the Phone App and request Optus send one out. They should do it for free but might charge a nominal fee.
PS That modem only seems to do 30Mbps wired. A new modem would increase that to 100Mbps potentially.