I want to automatically power cycle my Internet router in case it gets unresponsive. HA is installed here on HA Yellow, Yellow is connected to the router by Ethernet cable. Matter & Thread in general operate as expected and have no real issues.
Sometimes the Internet router gets unresponsive (e.g. surges on the power line). So I would like to reset it by a power cycle. To do so, I added an Eve Energy Thread-Matter plug where the power supply of the router is plugged into. Eve Energy generally works as expected with HA. Now it should be used to power cycle the Internet router.
The automation for that is setup and the traces show it is triggered and runs as expected when the router is not responsive for a certain amount of time.
I tried dry runs of that scenario by using the manual switch button on Eve Energy to manually power down the router.
The auto power cycle does not happen. Looking at the logs, I can see the Eve Energy switch is set to unavailable in HA shortly after the router is off.
So I assume, Thread-Matter devices are dependant on an active and responsive local network. Right?
If so, is my only option to use other smart plugs, e.g. ZigBee if Z-Wave based one for that scenario?
What are you using as Thread border router? If it’s the HA Yellow you should not need any LAN connectivity to control the Eve Energy from an automation running on the HA Yellow.
HA Yellow, is used here. And I was very astonished that Thread devices became unavailable when the router = WiFi was off.
The most likely cause is because your router is providing IPv6 addresses through DHCPv6 or SLAAC to your network. When your router goes offline, the devices on the network loose their IPv6 addresses. Does your router provide an ULA or global IPv6 address range to devices?
Now that said, I assume you run the Matter Server as well as the OpenThread Border Router on Yellow? In that case, Yellow itself should actually be able to use the mesh local IPv6 address as source address 
So I am not entirely sure why even Yellow isn’t able to communicate in that case. You’d need to look into the logs and use Wireshark or similar tools to look at the traffic to better understand what is happening.
Technically, it should entirely possible to make Thread as “standalone” as Zigbee or Z-Wave.
Yes, both OTBR and Matter Server on Yellow. And yes, my assumption had been Thread should continue stand alone there.
I will try it again with the newest HA / Matter / OTBR releases and see if the issue continues. I had used Wireshark in the past. But do not know how to do so for OTBR on Yellow. Any hints?