Its cheaper to buy a 5G alternative to NBN

Its twice as fast, and in some cases, can save up to nearly a quarter if the price.

Whats the difference? Why should I stick with NBN?

And why are those "super fast 500mb/s, $30 per month" plans not available at my place. What am I missing, and can I upgrade?

IInet is a prime example of this https://www.iinet.net.au/internet-product/broadband/nbn/plans/fibre