Hi Britgirl and thanks for looking into this, I have rebooted the VM and also restarted the nx server, it is still running on port 20001 after the reboot:
$sudo lsof -i :20001
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
nxserver. 12446 nx 15u IPv4 107411 0t0 TCP localhost:47002->localhost:20001 (CLOSE_WAIT)
nxserver. 12908 nx 8u IPv4 114166 0t0 TCP localhost:20001 (LISTEN)
Attached is the content of my server.cfg from /etc/NX/server/localhost/server.cfg :
######################################################################
# #
# Copyright (c) 2001, 2019 NoMachine, http://www.nomachine.com. #
# #
# All rights reserved. #
# #
######################################################################
#
# Specify the path of base directory where the NX software has been
# installed.
#
ServerRoot = “/usr/NX”
ServerSlaveBase 30000