@steevi2307 

As a fellow Synology NAS owner I would suggest you google and read "What stops my Synology NAS from entering Hibernation?" from Synology Knowledge Center. The list of possibilities is substantial and includes such things as "DHCP: You have enabled the Get network configuration automatically (DHCP) option on any LAN." 

In my case, I decided to just power off NAS at night when nobody uses it, by setting a startup and shutdown schedule. It is 8 hours downtime every night.

There is an opinion that setting NAS to sleep during the time when it has to frequently wake up could be detrimental to hard drives longevity. People who hold that opinion (including me) recommend to either not use sleep/hibernate at all or power off NAS for extended time, such as overnight.

  @steevi2307 

One more thing: if you can configure your Deco mesh to run in Access Point mode, try that and see if NAS goes to sleep now. Switching a Deco to Access Point (AP) Mode

  @Alexandre. 

Hi, actually, it's already in access point mode.

The NAS has a fixed ip.

And the Nas hibernation worked good before.