Instructions to update the IP address of a server:
Over the Datto web portal, go to the Protect tab.
Scroll down the page and find the server in question.
Click on the Configure Agent Settings on the left.
Scroll to the bottom of the page to the Rename Agent section.
From there enter the new IP address or new hostname and click on Rename Agent. Then you should see the communication repaired and you should be able to start a new backup right away.