DS-Lite configuration is not working with the OpenWrt router behind Fritz!Box

Hi OpenWrt community

Unfortunately, I came to know about the OpenWrt bit late, several weeks ago and I am really happy with all the customisations that I can do with that.

Fortunately, my own connection is still utilising the dual-stack which means I am getting a public IPv4 address and a public IPv6 prefix. But several days ago one of my friend's connections was upgraded to DS-Lite which means he is not getting any public IPv4 address at all. According to the ISP most probably my connection will be upgraded soon or later. Therefore I have to be ready with that.

I was using Fritz!Box as my main modem router before getting to know the OpenWrt and I want to use the Fritz!Box further as the interface for the DSL for several reasons even though I am aware of replacing the Fritz!Box with a modem or use the Fritz!Box with bridge mode.

My current setup is as follows:

Internet (1&1 with DS-Lite) → Fritz!Box → OpenWrt router (exposed host) → Lan

The internet is provided by the 1&1 and I am (in this case my friend) getting only a public IPv6 address prefix and not any public IPv4 address.

The Fritz!Box is having only one client which is the OpenWrt router as an exposed host which is more or less equivalent to DMZ.

The Raspberry Pi based OpenWrt with the second USB 3.0 ethernet port which is facing the WAN in my case the Fritz!Box, and the built-in ethernet is connected to a 24 port Netgear managed switch.

My setup is as follows and I have created the wan6_4 interface manually.

config interface 'loopback'
    option device 'lo'
    option proto 'static'
    option ipaddr ''
    option netmask ''

config globals 'globals'
    option ula_prefix '[hidden]'
    option packet_steering '1'

config device
    option name 'br-lan'
    option type 'bridge'
    list ports 'eth0'

config interface 'lan'
    option proto 'static'
    list ipaddr ''
    option ip6weight '200'
    option ip6assign '60'
    option device 'eth0'

config interface 'WAN'
    option proto 'static'
    option netmask ''
    option device 'eth1'
    option ipaddr ''
    option ip6weight '100'
    option ip6assign '60'
    option gateway ''

config interface 'WAN6'
    option proto 'dhcpv6'
    option device 'eth1'
    option reqaddress 'try'
    option ip6weight '200'
    option ip6assign '60'
    option reqprefix '56'

config interface 'wan6_4'
    option proto 'dslite'
    option encaplimit 'ignore'
    option tunlink 'WAN6'
    option peeraddr '::' // A real value

The WAN is disabled during the test and unfortunately, I could not ping I will appreciate any help.

Thanks in advance