Hi, new poster here, any advice appreciated!

Have just moved to Highbury in Wellington, and connected to Bigpipe's Starter Fibre, for which Bigpipe provided a Spark Smart Modem 3. 

The ONT (already installed by previous occupants) is at one end of the house, on the lower floor (house is built on a steep section, so lower and upper floors are 'offset', rather than being directly up and down.

When I first set it up, I found that I could get a download speed of more than 300mbps when close to the router, but upstairs (where living room is) the speed dropped to around 3mbps, with it sometimes dropping out. 

I have a Fritz! Repeater 3000 that I bought from 2degrees a couple of years ago for our previous house, so I rigged that up last night in mesh mode (I think, at least I followed those instructions) and put it in the living room. I can now achieve the lofty heights of 30-40mbps upstairs, so a lot better than before, and while the Wi-Fi connection is pretty constant, the internet connection sometimes drops out completely, so it's not exactly great.

The advice I'm after is how to improve the speed and reliability upstairs, and/or answers to the following:

 

1. Am I better off ditching the Fritz and buying a Spark Smart Mesh 2 (i.e. would that work better than the Fritz, with it being from the same provider as the router)?
2. Might I have the Fritz repeater set up too far away from the router? (There isn't really a good halfway spot, except maybe halfway up the stairs, but there's no power there).
3. Am I better off upgrading the router itself instead of attempting to extend the network with a mesh unit?
4. Is there a strong argument for running a long ethernet cable from the router to upstairs, and if so, what do I plug it into?

I hope that's clear enough - I don't know a lot about networks, etc. It doesn't need to be ultra-fast; just stable, reliable, and quick enough for streaming services, etc.

Thanks in advance.