Turning on mobile data switches from 4G(LTE) to 3G then back to 4G(LTE)

Hello,

I'm from Australia and I currently have a iPhone 6. For us the 4G indicator means you're using LTE. I have a issue that has been occurring from the start of this year (since iOS 11 to iOS12) where if I turn on mobile data I see it stay on 4G for 2 seconds, switches to 3G and stays there for 2 mins then switches back to 4G.

Its really annoying since the previous behaviour was that if you turned on mobile data you'll be on 4G instantly (provided you have good signal strength of course). I've already checked the mobile towers, called Optus (my carrier) about settings, got a new sim card, reset my iPhone, reset the network settings for my iPhone, when to a different state (Melbourne) thinking it was just Sydney affected and I was wrong.

So if anyone has any ideas or feedback to fix this that would be greatly appreciated.

Thanks

Michael