:local piholedown [/ip firewall nat get ([find where comment="PiHole-DNS"]->0) disabled] 
:local piholeDNS "10.0.0.1"
:local testDomain "www.google.com"
:local token "XXXXXXXXXXXXXXXXXX"
:local chatid "YYYYYYY"

:if ($piholedown = false) do={
    :do {
        :resolve domain-name="$testDomain" server="$piholeDNS"
    } on-error={
        /ip firewall nat
            disable [find comment="PiHole-DNS"]
            enable [find comment="Google-DNS"]
        :delay 2000ms
        /tool fetch "https://api.telegram.org/bot$token/sendmessage\?chat_id=$chatid&text=Pi-Hole not working! Changed DNS to GoogleDNS." keep-result=no
    }
} else={
    :do {
        :resolve domain-name="$testDomain" server="$piholeDNS"
        /ip firewall nat
            enable [find comment="PiHole-DNS"]
            disable [find comment="Google-DNS"]
        :delay 2000ms
        /tool fetch "https://api.telegram.org/bot$token/sendmessage\?chat_id=$chatid&text=Pi-Hole working! Changed DNS." keep-result=no
    } on-error={}
}