The right way to change this value is with qvm-features like so:
# Example with 192.168.0.100
qvm-features <qube> net.fake-ip 192.168.0.100
Which returns:
qvm-prefs <qube> visible_ip
192.168.0.100
Doc: qvm-features – manage domain’s features — Qubes Admin client mm_9c733d84-2-ga5ea121 documentation