How to put ipV6 to work correctly with DHCP and DNS in the local LAN?

I am using openWRT in my local lan as the main router to serve IP leases and as a DNS server for the local LAN.

It was working correctly till now, serving IPv4 leases under IPs

I have a local domain called 'casa' with some static leases (like the pointing to ip and to

It used to work correctly and nslookup in windows machines solved names likee correctly.

But from some time ago I am having dns troubles.

when I try to use nslookup in my windows machines I get a dns error saying there is no IP register available for

Trying to investigate it, it seems that now windows prioritize IPv6 over IPv4 addresses for all tasks.
If there is a IPv6 lease in the machine and a IPv6 DNS it uses it instead of ipv4.

The IP ULA prefix of my router is something like fdb4:xxxx:xxxx::/48 (I don't write it completly as I don't know if there is some security issues).

But the router is not serving DHCP leases, the windows machine gets the IPv4 correctly from DHCP server but it shows a IPv6 lease like fd84:a45b:c21::be2 which does not match the ULA prefix.

The ipv6 DNS server is stablished as fd84:a45b:c21::1 while the IPv6 of the interface of the router is fdb4:xxxx:xxxx::1
So ipV6 is not working and windows is using an auto configured IPv6.

I know it is possible to disable ipV6 in windows, but that will make me change the default configuration in all windows machines and I prefer to avoid that.
It would be error prone each time I connect one machine or device and it is clear that ipv6 will be the path to follow in the coming years.

I have tried to activate ipv6 server in the router with no luck.

In the lan interface, under the dhcp server tab there is a 'IPv6 settings' subtab.
I have tried to activate RA-service and dhcpv6-service changing it from disable to server mode (as the router would be the only dhcp server in the lan) but it is not possible to select that option, it is greyed.

How do I activate dhcp v6 server and configure it correctly to serve leases and solve dns in the local domain (.casa) and forward dns queries for other domains?

Thank you very much for any advice.