trying to use Matter/Thread devices with a Sonoff Dongle-Max

Will need to leave IPv6 disabled for now on the RBR. There is a standing known issue with IPv6 currently that is being worked on and developed to get IPv6 working in FW. Not sure when the fix will be forth coming. All I know its being actively worked on currently at NG. 

What you might want to try is find you a smart or managed LAN switch that has built in IPv6 protocol support. Connect it behind the RBR and connect ethernet devices to this switch for some IPv6 support on the LAN side. You could even connect the RBS to this switch as well and might help with IPv6 pass thru to the wireless devices connected at the RBS.

For the most part IPv6 isnt needed by most wifi or LAN devices. Some may be specific however most should work find with out IPv6 enabled. 

Until NG gets IPv6 fully working, you'll need to leave it disabled on the RBR for now. Nothing else we can do for now.