Extend add-on trial for Jira Datacenter

Go to my.atlassian.com and select Licenses. Find the Mobility for Jira add-on then click on New Evaluation License (see screenshot). Copy/Paste the new evaluation key into Jira/Manage Add-ons/Mobility for Jira

You should be able to extend it up to 3 times. If that option isn't there, just open up a ticket at support.atlassian.com. They will be able to extend it. It may take about 24-48 hours.