Two things first: 1) The DS216+II is not officially designed by Synology for a RAM upgrade. However, numerous manuals certify that it can be done - but you do it at your own risk.
2) The DS216+II is said to be a bit finicky about accepting RAM bars, so I would stick with a product that has been successfully tested by other reputable sources.
Here are two links:
https://www.youtube.com/watch
https://www.datenreise.de/synology-d...
have fun