I'd like to create a little DIY NAS with a Optiplex Micro that will be placed under my router and wired via ethernet. The NAS will serve as a Jellyfin Media Server Network share. Latter one is not very important for me, I mainly care for media server.

What I'd like to use my Jellyfin server for: - Streaming 4K Blu Ray Rips from my Shield TV Pro - Being able to stream (transcode) my stuff on my phone while in bed

So far, so easy. Now the problem: I live together with my mother in a rented two room flat, the Internet Access is in her room. She is very sensitive for noise (e.g. she can't stand ticking clocks when trying so sleep). I worry that she might be annoyed if a Micro PC is running 24/7 and will mostly be used in the evening (when I got time to watch movies and she prefers to go to bed). So I cannot connect any clients via ethernet, just WLAN. Which should be fine since the router is quite new and supports high bandwidth. At least I hope. If not, the whole plan is fucked from the beginning. But the main Problem is the noise. Man, life sucks sometimes with its little annoyances.

As an alternative: Is it possible to hook up an old router (including a little switch) I have laying around to my NAS and then connect the Shield TV to this router via ethernet? This would be not connected in any way to my router with internet access. Is it possible to still access any online-streaming on my ShieldTV via WLAN to my actual router, so I can watch YT, etc.? [I mean this in a comfortable way! So that the shield is simultaniously connected to the local LAN from router/switch B AND the WLAN/Internet from Router A]. And is it possible to access movies on my phone via the local LAN/WLAN on the second router, too?

Could it be possible to connect this 2nd router to my main router (the one with internet access), so I can have my NAS hooked up to the internet (e.g. for self hosted cloud storage for me to play around with)?