@mikeyh Hi Mike,
thanks for question. This is not about REXYGEN settings but Linux/Raspberry Pi OS settings.
The easiest way is to configure the RPi to have a static IP address.
Other method is to configure RPi to wait for DHCP server and if it won't receive any IP address then use fallback to static IP. This approach combines best from both static and DHCP.
Try to Google the keywords from above to get the up-to-date instructions.