At openwrt they seems to found an other issue why it may not works properly.
https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=bee7ff7cf34dfe66cf71c2e28faa1a03dbfc334c
Commit message:
Some broken ISPs (e.g. Comcast) send DHCPv6 packets with hop limit=0.
This trips up the TTL=0 check in the PPE if enabled.