Reply To: Can a Pi Zero get Ethernet through its mini-HDMI port (HEC) for a sensor node?

I’m a facilities guy, not an EE, but I chased this exact idea for a lobby display last year and burned an afternoon on it. The Pi doesn’t implement HEC. On HDMI that’s pins 14 and 19, and the Pi has no Ethernet PHY behind them, so there’s nothing to switch on in software. No overlay will fix it.

What I ended up with, and what I’d try in your order:

1. A micro-USB to Ethernet adapter on the Zero’s USB port (not the PWR one). ASIX AX88772 based ones work out of the box, eth0 just appears. Mine was about ten dollars.
2. USB gadget mode: the Zero pretends to be a network card to a laptop over the USB cable. Add dtoverlay=dwc2 to config.txt and modules-load=dwc2,g_ether after rootwait in cmdline.txt. Then the laptop has to share its connection.
3. Swap for a Zero 2 W if Wi-Fi ever becomes allowed.

Once eth0 is up the MQTT side is the same as any other Pi, TLS on 8883 with the device token.