Looks like IPv6 is currently disabled for the VLAN shown ..... it should be on DHCPv6. Is that screenshot correct?
@aaiyar - This looks like moving the hub back to the 192.168.20.x subnet will be the fix...from Unifi forums:
IPv6 is likely not necessary so long as the devices only need to communicate intra-VLAN. If they need to communicate inter-VLAN, then you may need to enable v6 at the gateway.
Wife and I are leaving shortly for some errands/appointments, I'll get back after I have time to make the change and confirm.
Thanks for everyone's help w/this.
You are going inter-VLAN, so you need to enable IPv6. Or move the hub to the other VLAN - whichever you feel more comfortable doing!
This...quicker/easier, and re-enables a bunch of quick-access shortcuts I had created and didn't want to have to recreate.
IMO you can safely enable IPv6 and it should not hurt anything. If anything it will improve your internet experience. This site will tell you if its working once enabled. https://test-ipv6.com/
Not sure what setting to use though especially for your router. For my router I have it set to "Native" and it gets an IPv6 prefix from the ISP which then gets used for all the devices. That might be the same as the SLAAC setting for yours.
If you have an internal DNS server it might mess with that a little bit and need some extra configuration. Thats about all that I have had to mess with. All I really did was turn it on and go with it. I have had it enabled in my LAN for years.
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The Unifi IPv6 settings look a bit more complicated so I'll leave that for another day. Today is not a good day to blow things up. 
Still have to finish my basic migration tasks, including FW rules, reserved IPs, WireGuard, etc. Enough on my plate for now.
First task (after this) get reserved IPs back in place so my wife's shortcuts to stuff on our NAS aren't all ferklempt (sp?). 
A little bit of Googling and I have determined you probably want the SLAAC setting. If you have to pick an Interface Type use the Prefix Delegation which will get the prefix from the ISP according to the pop up tip. SLAAC basically just allows the network to self manage the IPv6 and you should not really need to do any other configuration for normal stuff. Not sure about VLAN config so it works across those.
Even if it does not work it would not cause any sort of mass chaos. On a slow day just try turning it on and then try the ipv6 test site. If you cannot get it working right you can always just shut it back off again with no harm done. Its way less scary than you think it is.
That is definitely the plan, you've piqued my interest about enabling IPv6.
Just need to find the slow day...
By chance have you tried just powercycling the matter device, or the hub? I have seen that when matter communication is gets messed up just pulling power to the device and letting it restart will help
Thanks for the suggestions. I've power cycled both the hub and the matter devices (one of the devices, anyway), and that didn't help.
This is what is needed. I have had access to Matter devices for some time. IPv6 has been disabled on my external interface and the Vlans for my devices since creation.
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Just back to confirm that getting the hub on the same subnet solved the issue. Matter devices now cooperating.
Thanks very much to everyone.
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