I was in Melbourne/Sydney recently using a US model Nexus 6 (flashed with Nitrogen OS which is a Android Pie based ROM).
I was roaming most of the time on Vodafone since my SIM is Vodafone NZ (though I guess I could have chosen a different carrier)
The US Nexus 6 supports the following LTE bands
LTE band 2(1900), 3(1800), 4(1700/2100), 5(850), 7(2600), 12(700), 13(700), 17(700), 25(1900), 26(850), 29(700), 41(2500) - XT1103
and from what I can find, Vodafone AU uses
850MHz (B5) 1800MHz (B3) 21000MHz (B1)
So theoretically I could have picked up LTE bands B3 or B5 but during my entire time there, I am sure I only saw the phone display H+. I am wondering if it's either really a phone compatibility issue or the ROM has problems with its modem.