If you are receiving your IP from your ISP through DHCP there is a good change you are also receiving the DNS servers. By default MikroTik is going to propagate them to your LAN. This is how you can override it:
[user@MikroTik] > /ip dhcp-client set use-peer-dns=no [user@MikroTik] > /ip dns set allow-remote-requests=yes servers=126.96.36.199,188.8.131.52
Here I used OpenDNS servers but of course you can use whatever you’d like to.