Please visit the Remote.It downloads page and download the Device Package for Synology to your computer.
Access the Synology NAS using the web interface or Synology client application. The following pages show DSM 7.1.
Open the Package Center.
Click on the "Manual Install" button.
Select the file you downloaded, then click on "Next":
The following warning is normal. Click on "Agree".
Accept the License Agreement, then click on "Next".
Find the license key in the Accounts page of the Remote.It Desktop or web application, then copy and paste it into the next dialog.
Now click "Done" on the final screen.
Confirm in the "Installed Packages" screen of the Synology Package Manager that Remote.It is shown.
In the Remote.It Desktop or web app, confirm that your NAS Device is shown.
Next, add some Remote.It services.