As I am sure you are aware, if you have a static IPv6 prefix (ie address block) assigned then there is no advantage taking info from the ISP via relay DHCPv6 over the WAN, unless they give out some other juicy good-to-know info too. It is arguably just one more thing to go wrong.
You might want to check what you are doing with the RA M and O flags too. I got this wrong myself to begin with.
Btw, does IDNet use PPP?