Two ways of switching DNS via MikroTik script if PiHole fails.

pihole-peeper-upstream.rsc

# Mikrotik is set as DNS resolver (proxy).
# Changes upstream DNS config if PiHole fails.
# Tries to resolve google via pihole
# If it fails and DNS currently is pihole, sets fallback DNS as primary
# If it succeeds and DNS currently is not pihole, sets it as primary
# Notifies owner via his/her telegram bot

{
    :local currentDNS [/ip dns get servers]
    :local primaryDNS "192.168.1.5"
    :local fallbackDNS "1.1.1.1,8.8.8.8"
    
    # Telegram API stuff (https://stackoverflow.com/a/32572159)
    :local chatID ""
    :local botToken ""

    :do {
        :resolve google.com server=$primaryDNS
        if ($currentDNS != $primaryDNS) do={
            :log info "PiHole ($primaryDNS) back up"
            /ip dns set servers=$primaryDNS
            /tool fetch "https://api.telegram.org/bot$botToken/sendmessage\?chat_id=$chatID&text=Pi-Hole is back. Switching to $primaryDNS." keep-result=no
        }
    } on-error={
        if ($currentDNS != $fallbackDNS) do={
            :log warn "PiHole ($primaryDNS) down. Falling back to $fallbackDNS"
            /ip dns set servers=$fallbackDNS
            /tool fetch "https://api.telegram.org/bot$botToken/sendmessage\?chat_id=$chatID&text=Pi-Hole not working. Switching to $fallbackDNS." keep-result=no
        }
    }
}

pihole-peeper.rsc

# Changes DHCP DNS entry.
# Tries to resolve google via pihole
# If it fails and DNS currently is pihole, sets fallback DNS as primary
# If it succeeds and DNS currently is not pihole, sets it as primary
# Notifies owner via his/her telegram bot

{
    :local currentDNS [/ip dhcp-server network  get number=0 dns-server]
    :local primaryDNS "192.168.1.5"
    :local fallbackDNS "1.1.1.1,8.8.8.8"
    
    # Telegram API stuff (https://stackoverflow.com/a/32572159)
    :local chatID ""
    :local botToken ""

    :do {
        :resolve google.com server=$primaryDNS
        if ($currentDNS != $primaryDNS) do={
            :log info "PiHole ($primaryDNS) back up"
            /ip dhcp-server network set [find] dns-server=$primaryDNS
            /tool fetch "https://api.telegram.org/bot$botToken/sendmessage\?chat_id=$chatID&text=Pi-Hole is back. Switching to $primaryDNS." keep-result=no
        }
    } on-error={
        if ($currentDNS != $fallbackDNS) do={
            :log warn "PiHole ($primaryDNS) down. Falling back to $fallbackDNS"
            /ip dhcp-server network set [find] dns-server=$fallbackDNS
            /tool fetch "https://api.telegram.org/bot$botToken/sendmessage\?chat_id=$chatID&text=Pi-Hole not working. Switching to $fallbackDNS." keep-result=no
        }
    }
}