In your modem under advanced setup>Wan IP addressing>Under 4. (Static DNS address) Primary and secondary then save. Reboot the modem.
If you’re using the DNS as a geolocation workaround. Keep in mind if you have IPV6 enabled most times till will render your “spoof dns” useless (will geo locate you) IPV6 will need be disabled and only use IPV4.
Under advanced settings>IPv6 wan settings>Set the IPv6 state (disabled) and apply.