My house can only get FTTN type NBN and only gets just over 50Mbps (maybe 55) even on a 100Mbps plan.
As I'm away from home over winters I've been using Boost sims with the Cashrewards cash backs for over a year now either in my phone, or in an old Telstra modem I bought for under $50 a few years ago. That gets me a decent speed most of the time (25-40 Mbps). It's a cheap way to get acceptable internet with only the slight hassle of having to activate a sim every few weeks.
I've bought a dual sim 4x4 MIMO 5G modem/router and a spiffy 5G 4x4 MIMO antenna (way overkill) to see if I could get a better connection. There's a 5G Telstra & Optus tower with line of sight about 1.5 km away from my roof. I can also use the modem in my motorhome, so I've got a one year Amaysim sim as well in the second slot to have an Optus network option too (although that's 4G only, but that should be fine camping).
I got pretty good speed just having the modem sitting looking out the window using the little stick antennas that come with it. Around 80-90 Mbps on 4G (couldn't get 5G).
After installing the roof antenna (lined up straight at the tower visually) a few speed tests showed 115 down & 90 up on 5G. Even 4G was about 105/40 Mbps. I need to test at some other times and via an ethernet cable to the modem (rather than wifi). Maybe try it not pointing directly at the tower too just in case it's better offset a bit.
Pretty big initial outlay with modem, antenna, cables and mount. You could get good results with a simpler modem and antenna.
https://www.cudy.com/productinfo/1247700.html
https://www.rfi.com.au/LTE-XPOL-4X4-5G-D?quantity=1
(seems to be a rebadge of the Poynting XPOL-24)
(with 6 metre 240 cables)
FTTN should hopefully be pretty rare in the next year or two, I finally got my upgrade in December to FTTP and getting back on a gigabit feels like coming out of the dark ages. Address checker is here - https://www.nbnco.com.au/residential/upgrades/more-fibre.
This is a great guide for in the meantime to do a proper 4G/5G setup though, especially considering the threads on here about using a mobile phone or small 4G router for their home internet.