Sorry to hear you're facing some issues with the Mobile Network in your area.
I can see there's currently a fault on the tower between Earlwood and Marrickville. There's also a tower between Earlwood and Undercliffe that is undergoing maintenance.
@FonZPTo find where the towers are physically located you can do one of the following:
a) Install on your phone an app like "Aus Tower" https://play.google.com/store/apps/details?id=au.com.bitbot.phonetowers&hl=en_AU&gl=US
b) Enter your address in the following web site and search for the nearest tower via the map
c) Drive around until you spot the tower from the info Optus support has given you.
Good luck with a) and/or b).
BTW. There are also some towers near Canterbury station, so depending on where you are in Earlwood you may connect to these sometimes.