Harbourmaster,
My first mobile was a Motorola MicroTac (mini brick) on the Analogue network. It's still the phone that had the best range, on a trip Adelaide to Ballarat in the 90s, I had a signal the whole way. (I did have an in car with ext antenna) Now there are a number of black spots even on that road, terrain dependent (no ext antenna, but the signal strength shows on my vehicle console via bluetooth.)
About 6 years ago, 4G was the latest in the capital cities, many rural areas still operated 3G transmitters and the odd more remote towers may have even been 2G. At that time when staying in Dunolly, I started with a 3G mobile internet dongle and the locals told me their mobile tower was still 3G. A year or 2 later I changed to a 4G mobile WiFi modem and it was noticeably slower than the 3G dongle and still on a 3G tower.
My conclusion was that later system equipment, doesn't work as well on earlier networks or as well as the gear made for them. It would not surprise me that the cities get the latest i.e. 5G and the telcos slide the old 4G cells to rural areas to replace the 3G.
Let's face it, Capitals are going 5G and the 3 networks are phasing out 3G by mid 2024 (some earlier). I don't think they are replacing 3G in rural with 5G for some time, so it's logical to assume the moving of 4G cells as described.