Resolving IP address and connection reliability

One of my 4 ESPHome devices (Wemos D1 Mini) unreliably processes firmware updates.

Here’s the error I get in ESPHome when I try to update it.

Suspecting an mDNS issue, I enabled Local DNS Record on my Ubiquiti UDM-SE router, but it doesn’t seem to help.

Also, my network will often indicate that it’s connected while ESPHome indicates it is OFFLINE.

Any ideas for me to further troubleshoot this device’s reliability?

It is probably not a connection issue, but a boot issue.
Connect a serial monitor to the UART connectors on the ESPHome device and see what happens.

I wouldn’t know where to start with that suggestion.

If your ESP devices have USB connection then try to connect that to you desktop and then run a serial monitor, like Putty on Windows.
Linux have busybix or minicom and others and MacOS for sure also have some.

If you do not have an USB connection, then your ESP device have some UART pins for RX and TX where a FTDI adapter can be connected to get the serial monitor output.

I use PuTTY as an ssh client. So, you’re saying I can also use it to connect via USB? Once connected, then what?

Press reset on the ESP device and see the output.
The output is similar to the output in the ESPHome log window, but you can also get the log output before the network connection is established.

Most likely it is an mdns problem if the device is online and working with HA as usual. In my experience, the mdns record works short after an esp reboot but the mdns entry does not persist as it should.
If you cannot get mdns to work reliably, using a static ip works best (as the error states) because then esphome knows the ip and does not need the mdns to work.

Ping the IP address to test if the device is online.
You can’t use the Unify for anything, because Ethernet device will disconnect without sending an event message, so Unify will set a device as online when it is online for real, but also if it have seen it a while back and have not received any information that it should be offline.
It is an assumed state.

Ping should give you the right picture in the moment.