have you look under the sofa?
maybe contact optus via livechat for a replacement, i think for a lost one you have to pay for it, ~$20. If it is not working then optus will replace it for free 😉
or use the mobile app as suggested by @Ray_YC , however, you might need to the remote in the first place to set it up 😄 ..
try ebay or gumtree.