I recently installed secondary nas as backup in my family house. I connected with my home using tailscale.
In my family house there are 3 tp-link decos, connected via wifi, since I installed the nas, there are often issues with local network. The nas is idle, but it still somehow affects local network and slows down. Do you have any idea what is the issue there?

If you power the NAS off completely, do the network issues disappear? Wi-fi can be effected by all sorts of things and it sounds like you’re currently just going off of a gut feeling.
Honestly, my first step would be to get as much as possible on wired connections. This could be as simple as wireless spectrum congestion due to a neighbor getting a new wireless AP.
Then I’d be checking network logs for the synology. I would expect it to have some way to do that.
Beyond that, “network issues” doesn’t give a lot to work with. Is it complete loss of connection, slow speed? And can you quantify it with any network logs or diagnostics?
If it was just from adding one more piece of hardware to your network that is sitting idle, I would expect you to have seen it before now with say, an extra cell phone on your wifi.
Yes, powering off NAS makes issues disappear.
The network issues: the other devices were connected to wifi, but sometimes there was no internet connection or internet just slowed down. I will try to test it when I will be back there. For now I just asked to power off nas…
I havent seen any issues when there were more devices connected. So one person at home and nas powered on had worse experience than 6 people at home and nas turned off.