I have asked the same question from MT support. Unfortunately NAT-ing is not the right choice in all cases.
The way how you can set your primary IP address (pref. source) in this case is:
- Open terminal
- Type: /ip address print
You will see all your IP addresses and numbers in front of them. Your primary WAN IP (pref. source) is the IP with lowest number from same subnet on same interface.
Let's say your current pref source IP is the IP in position 3 and you would like to have the IP in position 5
**** WARNING YOU MAY LOSE CONNECTIVITY TO YOUR ROUTER IF YOU ARE CONNECTED USING IP YOU ARE CHANGING ****
/ip address set numbers=3 address=Put-a-dummy-IP
/ip address set numbers=5 address=Put-IP-from-position-3
/ip address set numbers=3 address=Put-IP-from-position-5
you can reboot your router to check if correct pref. source is chosen after restart, or simply disable and re-enable IP which you won't like to see as pref. source. In this case new IP which is in position 5. Since there is a "smart" algorithm for choosing pref. source it will automatically choose position N+1, so if you have WAN IPs above position 5, you will need to disable/enable each of them
I have open ticket with MT Support and advised few simple fixes/solutions for this issue. They are easy to implement so I hope we will see this resolved in one of future ROS releases.