I cant put my modem on bridge mode otherwise i do lose ipv6, its an ISP thing that wont change on near future,what i would like to do is to have openwrt on my router but with dhcp off, but force a custom dns to my computers and also wifi instead of the one we have from the isp, is it possible? How do i do it via Luci?
That's possible. I have a full dualstock connection from Ziggo with a Connectbox. Not using bridge mode due to loosing IPv6 connectivity.
In your connectbox, turn of the following features: IPv6 firewall and WiFi (both 2.4 and 5 ghz). IPv6 firewall must be disabled, it won't route traffic to OpenWRT when OpenWRT request its own prefix via IPv6 prefix delegation. OpenWRT should also be the only client on the Connectbox. With the IPv6 firewall disabled, IPv6 clients are directly exposed to the internet (the Connectbox configuration page won't be exposed). Create DHCP reservation for static OpenWRT address optionally. Put your OpenWRT router Don't turn of DHCP, otherwise DMZ won't work. Last, configure your OpenWRT router in the DMZ settings of the Connectbox. Now IPv4 and IPv6 traffic is routed to your OpenWRT router.
Make sure your OpenWRT router create its own network to serve wireless and wired clients, and connect the WAN port to your Connectbox.
Use scripts / DDNS to update IP addresses if you expose services to the web. Ziggo has long IPv4 lease times, I won't change for years. However, the IPv6 lease time is very short, so your prefixes could change after a router reboot.
Just to provide an alternative angle.
Using PREROUTING firewall rules, you can redirect all DNS requests (typically port 53) to a local DNS server instance like dnsmasq. Unless your devices use DNS over HTTPS or within their own tunnel, all requests will go to that instance.