How IPv6 DHCP relaying can be configured to ER605?

My operator LTE modem gives IPv6 addresses to my local network by passing through IPv6 DHCP information. For example, here are addresses for one computer:

    inet 192.168.8.2/24 brd 192.168.8.255 scope global enx000ec68e9793
       valid_lft forever preferred_lft forever
    inet6 2001:xxx:xxx:xxx:xxx:xxx:fe8e:9793/64 scope global dynamic mngtmpaddr noprefixroute
       valid_lft 6995sec preferred_lft 3395sec
    inet6 2001:xxx:xxx:xxx:xxx:xxx:75dd:3/128 scope global dynamic noprefixroute
       valid_lft 6393sec preferred_lft 2793sec
    inet6 fdf8:c39e:2875:dd00:20e:c6ff:fe8e:9793/64 scope global dynamic mngtmpaddr noprefixroute
       valid_lft 6995sec preferred_lft 3395sec
    inet6 fe80::20e:c6ff:fe8e:9793/64 scope link
       valid_lft forever preferred_lft forever

I connected the ER605 to this network and configured its WAN IPv6 part as automatic, so it received IPv6 address and IPv6 DNS information from operator. I have tried all combinations on LAN IPv6 side I imagined, but still I could not get IPv6 addresses passed to clients behind it.

Simple question: how IPv6 settings should be defined in order to pass IPv6 DHCP queries through the ER605?

In two another sites, I have OpenWRT router behind similar LTE modem. On those, OpenWRT LAN IPv6 settings are as follows: