There are only 3 networks in Australia at the moment, until TPG launch theirs. Vodafone, Optus and Telstra. If you're in Sydney, you should be getting good coverage, whichever network you're on. Vodafone's speeds and coverage are just as good as any other provider as reported in the news yesterday.
http://www.smh.com.au/technology/technology-news/revealed-the-fastest-telcos-and-cities-in-australia-for-broadband-20171108-gzhmyb.html
if you're phone is old, that may play a part in it. The ariels can get damaged if you drop them. Might be time to consider an upgrade. Get on eBay for second hand or go for a cheap $200 or so Android device - you'll get 90% of what you will with a new Galaxy phone for a fraction of the cost. And your signal strength may well improve.
In sydney, I'd recommend Vodafone or one of their resellers - you'll get more data for your money. There are comparison sites which can help you get the most for your money. If you provide some idea of the amount of data you need, you can decide what provider is best for you. Optus have a bunch of Xmas offers on, for example, at the moment.