WAN IPv4 to LAN IPv6

Hello newbie here,
my point is to get rid of double nat, actually my setup is this:

ISP Router (DCHP IPv4) > OpenWrt Router DCHP IPV4/6 > PC

OpenWRT Router is LAN conected via ethernet port of ISP Router to his WAN ethernet port, creating 2 subnets double Nat.
I cant get rid of ISP router beacause Voip and do they do not offer Ipv6, my question is if its posible to use Ipv4 in ISP Router to reach OpenWRT Router and in OpenWRT give Ipv6 adresses only to a pc and get rid of second nat?

Thanks so much

Convert OpenWrt to a dumb AP, or add a route on the ISP router to your downstream subnet via the OpenWrt IP and disable masquerading on the WAN firewall zone on OpenWrt.

Thanks for the reply, ill try what you point me but the ISP Router is so restricted I cant make static routes.

Does they Ipv6 solution too much complicated to the subject or its imposible to make it works?

Thanks again for the help, much appreciated

IPv4/IPv6 transitioning is not trivial, and each IP protocol generally works separately including routing and connectivity, that's why IPv6 cannot hide problems of your IPv4 setup, moreover IPv4/IPv6 transitioning is also a form of NAT, so it doesn't solve the double NAT problem.
Unless you plan to install OpenWrt on the ISP router, then get a public IP address, set up IPv6 tunnel broker, forget about IPv4 issues and use only IPv6, however you will likely face bandwidth and latency issues due to non-native IPv6 setup.

Ok first thanks for the replies, Im a newbie in this subject and I was just wandering but its more complicated than I expected, anyway I finally figured out how to make a static route in ISP router pointing my downstream subnet, can you help me figure out the next steps please?

Next step should disable masquerading on the WAN firewall zone on OpenWrt, but just a question before this step, why not make a static route the inverse way from Openwrt router to ISP router?

Thanks so much

The downstream router already has all the necessary routes, meanwhile the upstream router has no route to the subnet behind the downstream router, which is required with disabled masquerading.

I just try disabling masquerade on WAN firewall zone and restarted network but it didn't work.
I still have ip address from ISP router but no connection with upstream zone. Ping not responding to any website.

Also i try to add a static route in OpenWRT router to pointing to ISP Router but also didn't work.

OpenWrt needs no extra routes, unlike your ISP router.
Post the routing config/status from the ISP router.

Network adress --Subnet Mask --Gateway ----Interface IP------WAN
Public IP ----------- --------------------WAN -> ISP Router, LAN Upstream subnet -> OpeWRT Router, LAN Downstream subnet -> OpeWRT Router IP

LAN2 was made for setup a static route to OpenWRT Router beacuse of ISP Router rectrictions
Thanks for the help

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Be sure that LAN2 is just an alias for LAN, otherwise it is likely to fail if LAN2 is a separate interface.