Why does my ethernet ESP32 controller keep dropping off the network after a few days?

I had almost this exact setup, ethernet ESP32 driving a bank of relays, and it did the same random offline dance until I swapped the power supply. Cheap 12V adapters from a drawer are prime suspects. Relays switching all pull current in the same instant, a weak adapter sags for a few milliseconds, and the board browns out. Your wifi boards survive the same dip because the radio just retries the connection. The ethernet side tends to drop the link and sit there dead until it gets a full power cycle, which is why the wifi ones look bulletproof and this one looks flaky. Here's a test that narrows it down fast: next time Home Assistant shows it unavailable, go press one of your wall buttons that runs through the board. If the relay still clicks and the light still works, the board is alive and it's purely a link problem, which points away from the supply. If everything is dead, it's power. My money is on dead. Your multimeter probaly won't catch the sag since it's too quick, but you can try switching every relay at once from Home Assistant while watching the meter. Any visible dip means you've found it. Honestly the easier move is just to feed it from a bigger, decent quality supply for a week and see if the dropouts stop. A big electrolytic capacitor across the 12V input right at the board helps a lot too if you're comfortable adding one.

One other thing to check: many network switches have a power saving or green mode per port, and the ESP32 ethernet chips don't always get along with it. Random link loss every few days is the classic symptom. Try turning that off for the port the board is on, or just move it to a diferent port and cable and see if the pattern changes.

I get why you blame the supply, but I'd push back on the 'if the relay still clicks, it's the link' test. My ethernet board kept showing unavailable while the outputs carried on fine, and it turned out to be a flaky crimp on one of the patch cables. The physical layer can half die while the chip keeps running, so a clicking relay doesn't clear the wiring at all. Before buying anything, I'd swap in a known good cable and a different port on the switch first. Costs nothing and mine never dropped again after that.

I'd add one more angle: the LAN8720 PHY on these boards is sensitive to the GPIO bootstrapping pins, and if the relay board pulls the right pins at boot or during a glitch you can get a hang that looks exactly like a brownout. Worth checking that your relay inputs aren't tied to the same pins the ethernet module uses at reset. Also log the uptime entity, if uptime keeps resetting while HA shows unavailable you know the board itself rebooted rather than just the link dying, which separates power issues from network ones pretty cleanly.

Three weeks in is exactly when mine started doing this years ago. Turned out the transformer powering the whole cabinet was sharing a circuit with the boiler pump, and every time the pump kicked in there was just enough noise on the line to upset things. Back then I fixed it the old fashioned way, a seperate feed from a different breaker and a ferrite ring on the incoming cable. Never dropped again after that. Might be worth checking what else shares that supply before you blame the little board itself.