Telstra Smart Modem ipv6 DNS can't be disabled

[mimsy1]

I have set up a local DHCP and DNS server (pihole) and to ensure that it works as intended, disabled the DHCP server on my Telstra Smart Modem. This has worked in the sense that local clients are now using the IPv4 DNS server and not the modem/router. However, they are still being issued IPv6 DNS servers by the modem/router, despite the "Local Network-IPv6" option also being disabled.

While testing, I noticed that if I enable the "Local Network-IPv6" option, then select different DNS providers (Telstra or Google or Cloudflare etc), hit save, then disable it again, the new DNS providers persist!

Is there anyway to prevent this modem/router from continually sending out IPv6 DNS servers?

Modem model: Arcadyan LH1000 

Firmware: 0.07.05r

[cf4]

From my experience I have found that if you leave the local IPv6 state disabled permanently the clients will use there own IPv4 DNS server settings. In order for new DNS settings to take effect client has to be disconnected and then reconnected to modem.