I think the question is referring to a different field than the one you are...
Are you talking about the dst-address field as found in the 'General' tab in winbox, while I am indicating the dst-nat address in the action=dst-nat to-addresses=10.20.1.2...?
add chain=dstnat in-interface=MYCABLEMODEM protocol=tcp src-port=20-21 \
action=dst-nat to-addresses=10.20.1.2 to-ports=0-65535 comment="EXAMPLE PORT 20-21 for FTP" \
sure, and then add
add name="resolve-ip" source="ip firewall nat set EXAMPLE \
PORT 20-21 for FTP to-addresses=[:resolve yourdns.homedns.org]"
add name="resolve-ip" on-event=resolve-ip start-date=jan/01/1970 \
start-time=05:00:00 interval=12h comment="" disabled=no
This will check, resolve and reset your IP in the dst-nat rule at whatever interval you specify from whatever your dyndns registered name is - and voila! Automatic IP address-changing...
Surely that's what you meant...