If you are using DHCPv6, then the timer on the RA doesn't matter (for the WAN). Since you will get a lease time for the DHCPv6.
It sounds like there is an incompatibility between the DHCPv6 server on the Connect Box and the client on OpenWrt (these do exist).
As a work-around, you may consider restarting the WAN6 via a cronjob (in the middle of the night.