If serial console is connected, but not a monitor and keyboard, then first boot setup should be possible over serial console. If that is not the case, then you found a bug (at least in most of the recent releases, and R32.4.3 qualifies as recent). This script allows setup prior to flash:
https://forums.developer.nvidia.com/t/jetson-nano-all-usb-ports-suddenly-stopped-working/75784/37
The reason this change occurred is due to legal reasons in the state of California.