So I need some help. I'm moving into a new Accomodation (semi-temporary small place) where the owner shares wifi with me (sort of split house and main networking/fibre/router is in the other house with the owner). I need some advice on how I can easily/cheaply achieve the following:

1. I need ethernet for my nas

2. I would ideally prefer to have a private network so what ever happening in my network (including nas is not accessible outside my network)

3. I would ideally want to have my own wifi network 

What I already have:

1. Bunch of tplink deco x50 units from my previous place

2. Bunch of small 4-8 port switches

3. Fritzbox 7490 or similar (can't recall the exact model - it came with my previous 2degrees business connection)

What I need advice on is if there is any configuration using the fritzbox or the decos that would allow me to achieve my private network where the owner doesn't have access to anything happening in my network? Is there any sort of cheap ish device I can get (maybe even secondhand) that would allow me to have this? 

Please note: I'm not trying to get maximum speed or best latency or anything. At this stage, stability and decent speeds is enough. The main concern is the privacy and ability to have access to ethernet ports. Also, I could potentially ask the owner to plug in some devices near their network setup too (like a powerline ethernet kit or something) - so that's an option too. 

Any advice is appreciated. I'm a bit of a networking noob - I just know the basics.