If you want to run a script on your router here you go... Basically ensure that your "label" is marked as Dynamic on OpenDNS... then put the label, username, and password in the script. Run it every 10 minutes or whatever. If you wanted to you could make it check the current vs the old IP and only fetch if they differ...
#.* by RouterOS
# EntryDNS Update Script
:local openDNSUsername "USERNAME"
:local openDNSPassword "PASSWORD"
:local openDNSHostname "LABEL"
/tool fetch url="https://updates.opendns.com/nic/update? ... NSHostname" \
user="$openDNSUsername" password="$openDNSPassword" \