OpenWrt Forum Archive
https://forum.archive.openwrt.org/viewtopic.php?id=4304022.03.2013 · Here is the important bit, by default OpenWRT dhcpv6 will not accept the reply for autoconfiguration unless it has a subnet to delegate. config interface 'wan6' option proto 'dhcpv6' option ifname '@wan' option reqprefix no # only needed if your CPE does not delegate a prefix for you to route, you will need to relay to get ipv6 on the subnet.
OpenWrt Forum Archive
https://forum.archive.openwrt.org/viewtopic.php?id=6822524.10.2016 · Accept Router Advertisements even if forwarding is enabled. Try issuing "ip -6 route" on normal linux system and openwrt. In openwrt you see "proto static", in normal linux "proto ra". This is because on desktop linux forwarding is disabled and kernel accepts RAs. In openwrt routes look like someone manually added them.
accept_ra | sysctl-explorer.net
https://sysctl-explorer.net/net/ipv6/accept_ra09.12.2018 · Accept Router Advertisements even if forwarding is enabled. Functional default: enabled if local forwarding is disabled. disabled if local forwarding is enabled. Nb: per interface setting (where “interface” is the name of your network interface); “all” is a special interface: changes the settings for all interfaces.