a) Cancel the service and payout the $216 (max) or lower depending on how Optus calculate the cost of the modem over the 24 months. Once you payout the money you own the modem and can then sell it.
b) Try to lock the modem to specific bands in order to try and maximise the speed. Search for "LTE-H Monitor" (https://yescrowd.optus.com.au/t5/Mobile/Is-it-possible-to-Telnet-into-either-the-B525-or-B818-that-O...
c) Try a Beyond or Vodafone SIM that is on special at Woolies or Coles to see if they are better than Optus.
d) Do nothing and waste money.. Do not do this.
e) Check out whirlpool for external antenna's you put on the roof if you are in line of sight to an Optus tower. (https://forums.whirlpool.net.au/thread/9p8yvml9?p=120)
You can check the optus 4G coverage on the https://www.optus.com.au/about/network/coverage page, but be aware that this is will show best case and does not take into consideration interference caused by objects between you and the tower in allot of cases.