I got Optus Netgear AC800S and iPhone XR both from Optus.
I got better reception like Full Bar compare to 1 to 2 bars when using my Optus SIM Card in the Telstra Netgear Nighthawk M1 at my home.
Bars of different devices/vendors do not relate - is no set standard.
Things to be aware of - all 3 devices do carrier aggregation meaning more then 1 4g band at once.
One device may connect to band 28 700Mhz as primary and signal bar will be high.
Anotger may connect to band 3 1800Mhz as primary and of course 1800Mhz would show less bars etc.
Different devices have different antennas setups, nighthawk is the bigger device, gives it more room for antennas etc.