Standalone API connectivity issue

I am building a "MiniMaestro" that connects to my Flex using the network (see my QRZ.com page for pics and details). I have two versions - one that is Arduino Due/Ethernet Shield based and the other that is Teensy 4.1 based. The Teensy has a built in Ethernet port. So far I can adjust volume, frequency, AGC-T and a few other items. If I am plugged into my DHCP-served LAN at home, all is well. Both devices can be plugged into the network at the same time and both work great.

On outings in my 5th wheel camper, the Arduino Due unit works fine with a network switch (no router) by setting the ip to 169.254.5.16. The radio shows up as 169.254.5.220. The Teensy is running the same code (recompiled, of course) as the Due, but it does not work right. I have an ip of 169.254.5.25 set for it ( I did try others, including 16). If I turn the volume knob, it moves the volume a couple of notches, then just quits. If I hit the mute button, It will mute, then get lost. SmartSDR stays alive and continues to work perfectly. If I have both devices connected and active, the Due continues to work perfectly.

Sooo... To my question. Is there something that I need to do differently when running without a DHCP served address? Am I just getting lucky with the Due working and the Teensy not? Seems very strange that they both work flawlessly under DCHP, leading me to wonder if there is something that I need to care for.

Thanks & 73

Len, KD0RC