Applied model: 

NMP-302W, NMP580-W, EP1042T

1. Click "Advanced" to enter advanced settings page.

2. Click left mouse key three times in the right bottom area with interval of one second

3. Then Android main page will appear

4. Click "Settings" to enter settings page

5. Check "Unknown source" under "Security" setting.

6. Click "OK" to confirm

PS. Not all APPs can be supported as the devices have no GMS (Google Mobile Service) certificate.