I’ve had it up and running for 2-3 years now, works like a charm (In Adelaide). I think you’re confusing wifi with cellular frequencies. The rachio uses standard wifi 2.4ghz (or 5ghz) modes, same as any standard laptop, chromecast, etc. Otherwise the modem wouldn’t be able to connect with it.
Bigger problems Aussies have with it are the power plug, it comes in 120V wall worts, so you need to go to jarcar and buy another and swap the plugs over.