So, you have some of your house without wifi coverage? Is that on the 2.4G band as well? The 2.4g band has greater range than 5g band; you could sacrifice 5g speed for better coverage on 2.4g.
What is the "certain connection" only in that part of the house?
The easiest thing might be a wifi extender.
Or, you could use a longer ethernet cable between the NBN box and the wifi router.
If the NBN box connects into a telephone wall socket, a longer cable from NBN box to wall socket.
The last two depend on the layout of your house and available power points and how much extra distance the wifi has to go.
Kogan's wifi mesh is probably the cheapest compared to eero.
This should allow you to flood your entire house with the wifi signal you need to avoid black spots.