I'm only on RouterOS v6.33.1 on my hAP Lite, but it was under IP > DNS for me.
Your ISP usually has their local DNS servers populated there. Just click the drop down arrows and apply the DNS addresses where required. I personally, while still just testing, put CloudFlare's set as 1st and 2nd, Google's set as 3rd and 4th. and I left my local ISP's set as 5th and 6th just in case. Will obviously remove the unnecessary ones accordingly now, since I've been using it for several weeks and all seems well.