Can't connect via Matter to Nest Thermostat 4th Gen

Configuration Matter/Thread
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I recently migrated my HA setup from a Mac Mini to a UGREEN NAS, and miraculously just about everything worked, however at some point my Nest 4th Gen connected through Matter stopped working, became disabled and couldn’t connect. I went through a bunch of shenanigans right down to disconnecting all Matter links (including Google Home), which appeared to factory reset the Nest. I reconnected it with Google Home, but when I try to connect to HA via Matter (using the pairing in Google Home) it fails every time. I check the logs and see this kind of thing spammed:

2025-12-10 14:41:16.084 (Dummy-2) CHIP_ERROR [chip.native.EM] <<5 [E:1693i with Node: <0000000000000000, 0> S:0 M:195747843] (U) Msg Retransmission to 0:0000000000000000 failure (max retries:4)
2025-12-10 14:41:21.134 (Dummy-2) CHIP_ERROR [chip.native.SC] PASESession timed out while waiting for a response from the peer. Expected message type was 33
2025-12-10 14:41:21.135 (Dummy-2) CHIP_ERROR [chip.native.IN] SendMessage() to UDP:[fd4e:b16a:fa0:849:a817:df70:9b75:c561]:5540 failed: src/inet/UDPEndPointImplSockets.cpp:417: OS Error 0x02000065: Network is unreachable
2025-12-10 14:41:21.136 (Dummy-2) CHIP_ERROR [chip.native.SC] Failed during PASE session pairing request: src/inet/UDPEndPointImplSockets.cpp:417: OS Error 0x02000065: Network is unreachable
2025-12-10 14:41:21.136 (Dummy-2) CHIP_ERROR [chip.native.-] src/inet/UDPEndPointImplSockets.cpp:417: OS Error 0x02000065: Network is unreachable at src/controller/SetUpCodePairer.cpp:385
2025-12-10 14:41:31.600 (Dummy-2) CHIP_ERROR [chip.native.EM] <<5 [E:1695i with Node: <0000000000000000, 0> S:0 M:195747845] (U) Msg Retransmission to 0:0000000000000000 failure (max retries:4)
2025-12-10 14:41:35.054 (Dummy-2) CHIP_ERROR [chip.native.CTL] Discovery timed out

It looks like it’s trying to send to an IPv6 address there, which makes no sense to me as I’ve disabled IPv6 on my router, my NAS and in HA itself.

Worth noting that I did successfully connect a Shelly Relay via Matter just the other day, so Matter itself doesn’t appear to be broken.

I’ve googled the heck out of this with no luck. A similar topic mentions Attestation failures but that’s not what’s happening here.

Any advice?

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IIRC Matter is IPv6 only

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Well I finally got it working. For others with a UGREEN NAS, you need to enable bridging in your network settings for the adapter you want (I used Normal Bridging but Virtual Bridging might work too), then in the Virtual Machine app under Manage → Network → for the corresponding subnet change mode from MacVTap to LinuxBridge. You shouldn’t have to change your individual VM’s network settings (virtio/vnet-bridgeN).

Hopefully this helps someone out as I only managed it after a bunch of trial and error cobbled from pretty much every other forum post adjacent to this I could find.

One thing I did to help diagnose was ping the IPv6 address from my NAS (success!) then from the HA OS enter “login” then try the same ping (failure!) (can also do this from the terminal add-on), which established that I was supposed to be able to reach the IPv6 address from the VM - but couldn’t - even though I hadn’t enabled IPv6 anywhere.

Still don’t know why this was a problem for the Nest 4th Gen and not the Shelly relay. /shrug