https://www.youtube.com/watch?v=gVuabEckMMA

I am new to nixos too. As for flakes, it is little complicated at least for me. Vimjoyer video helped me understand little about flake. Here the link https://www.youtube.com/watch?v=JCeYq72Sko0 You can go through his nixos series. And there another YouTuber librephoenix. He has good learning material too. Here is link https://www.youtube.com/channel/UCeZyoDTk0J-UPhd7MUktexw

Try USB tethering from phone. I had similar issue with my first install. Updating drivers helped.

I had some issue with miui. There is nothing we can do.