During flash it is in recovery mode, and although it uses USB, there is no network involved over USB.
192.168.55.1 does not exist unless the unit is fully booted. Flash normally (and only) runs in recovery mode, and then as flash completes, the board reboots normally and this is when
192.168.55.1 shows up. The optional components (such as CUDA) are installed over
ssh, and so flash itself does not install CUDA or other extra software.
The use of
192.168.55.1 might require the host PC to be told to allow this (since it might not by default accept random USB network devices without someone manually allowing it). If you are at a stage when you think you should be able to use
192.168.55.1, then on the host PC monitor “
dmesg --follow”, and then disconnect and reconnect the host USB to see what shows up under
dmesg (might require powering with the barrel connector, otherwise the NX would be rebooting as well).