We have created a set of instructions with images you can see all the affected screens of the process. Official instructions from QNAP can be found here

Set up QNAP to allow unsigned app installs

1. Open the App Center
2. Select the settings cog

3. Enable setting to allow installation of without a digital signature

Installing an app manually

1. Open App Center
2. Select manual install icon

3. Follow installation wizard to select, upload, and install the Plex Media Server installer

4. Complete installation wizard

Signature warning when installing an unsigned app

Expected “No digital signature” warnings