In the Network | IPV6 | LAN section of the ER605 one can manually configure an address for the IPv6 DNS Servers

However, unless I use a full 128bit Global Unicast address (starting 2..) the ER605 reacts with a "Invalid Format" error message in red.

This means I cannot use a Local Link address (fe80...) or a Unique Local address (fd....) for my internal IPv6 DNS Server. No explanation of the required address format is given in any documentation.

Does anyone have a explanation as to why this error is issued or how I can circumvent it? Is there any documentation describing the validation being used?

The firmware seems to assume that all DNS servers are external, I would like to use an internal server running piHole so I need a stable (unchanging) internal IPv6 address as per a static IPv4 address.

Many thanks