OpenWRT - IPV6 - relay mode - nextdns - resolver not working

Hello - I am running into an issue with nextdns  when ipv6 relay mode is used in OpenWRT. 

When I don't have ipv6 enabled - dns resolution is NOT done by nextdns.  It works fine when ipv6 is disabled and only ipv4 is used.

Came across similar thread - but it's not clear how to override the DNS for ipv6 alone in OpenWRT in relay mode.

https://help.nextdns.io/t/60htjnn/ipv6-encryption-on-unifios

Has anyone experienced similar issue?

thanks