Hello,
I had the same issue and solved it by setting a firewall rule to block outside requests on port 53 UDP, The MK DNS server was acting as a public DNS server.
Your failure: As usual, in this cases, either you configured the firewall badly, or thinking you were the smartest you deleted the default rules that prevent this (and other things) from happening.
If my memory serves me right, the default settings for the MK firewall do not include a rule to block UDP traffic on port 53.
My sole oversight was not executing the correct takeover of the router. This was amid a sequence of transitions, starting with a change in the main provider. The former provider assigned IPs via DHCP, but then we switched to a provider that used PPPoE, and the existing rule was set for WAN1. Initially, this was fine, but after the transition to the new PPPoE provider, the rule ceased to apply, which went unnoticed until internet connection issues began to surface.