Some confusion here - you state you are using a private instance - why the references to TTN(V3 & associated infrastructure/console)?
If private you can set you own band decision - just make sure its supported and same at both GW, NS & end node and h/w supports. If using TTN then you need to follow the TTN AU freq plan(s)
Much discussed here:
And especially & more recently here:
Last time I looked whilst both continue I thought there was a growing concensus for AU915 but I guess been out of touch lately on that. As you say if your device doesnt support then maybe only AU923 an option - suggest double check with vendor(s) as may just be a firmware/config option vs major h/w issue.