Optus 3G (network not working) keeps overriding the working 4G network

Hi,

I’m currently in a somewhat remote location in Australia. I’m with Optus at the moment and there is a 3G network and a 4G network available. Last time I was here both networks were fine but this time the 3G network has 3 bars but the internet will not work via it. The 4G network has only 1 bar but is perfectly fine. It seems as though there is an issue with the nearby 3G tower but because it’s signal is stronger my phone keeps using it. I have 4G selected in settings, how to I stop my phone connecting to the 3G tower which won’t work?

(tried airplane mode and restarting already)

(iPhone 8 model A1863, iOS 13.3)

Thanks in advance