Check your manual and enter the SMS menu, which is like a BIOS/Firmware. There could be some self tests for selection, but for sure an option to show and edit the boot order.
Some old boxes get into the SMS menu via setting a metal key to the service position.
It would not be bad to have installation medias of old AIX version that are supported with that hardware. Also you should ask those from where you have it, what they did to the AIX on the disks. Or maybe there are different disks in there. You have the connections not us, so ask them