Wifi/LAN access/setup question

Wifi/LAN access/setup question

I live in a house that's been informally split in two. So, there is only 1 access to a landline.

The current set up is this:

1st floor - ISP router/modem (I don't know all the right terminology, so forgive me). This is also the floor with the connection to the landline.

It has multiple devices (around 10) only connected via wifi.

Ground floor (my floor) - I have a cable from upstairs connected to a netgear n300 (super old) that I have connected my things to (around 15 devices, which is growing.) I just plugged it in, it isn't set up as a repeater or anything, no idea what this situation is called. I also have my shield & PC connected by Lan from the n300.

1st floor just uses Facebook, Netflix & basic web browsing. I game, stream etc & I'm planning on getting a NAS for movies/TV.

The n300 I have is dieing & needs to be replaced. What would be the best set up for me to do? Should I continue with what I already have or mesh. I'm not too familiar with this sort of stuff, so any help is appreciated. Thanks!