After a long time of trying to figure out why my computers are from time to time using my Archer VR600 router as a DNS server I have found out that these seems to be no way of turning off DHCPv6 on this router, apart from disabling IPv6 altogether.

I had thought that the option on the Advanced > Network > LAN Settings > IPv6 between RADVD and DCHPv6 Server allowed me to decide which method to use to distribute addresses but it seems it just switches between the settings for both.  I turned off RDNSS on the RADVD page so that the router would no longer tell my clients to use it through Router Advertisments, but runnning Wireshark just now I noticed that the router is responding to DHCPv6 requests and giving its own address as the DNS recursive name server.  I don't want this as I have a separate DNS server I want to use.  Both servers are responding to DHCPv6 requests, and this results in a race condition where sometimes a setting is picked up from my desired DNS server and sometimes from this router.

Is there some hidden or obscure way to either disable the DHCPv6 server completely, or stop it announcing itself as a DNS server?