Is it necessary to click on the ad? Or is it enough to go to the web page where the ad is displayed?

With an old phone Firefox is indeed slow. Startup is slow, page loading is slow, even scrolling is sluggish. On Chromium browser the performance is a lot better if your phone is not that fast.

Why is Kiwi a terrible browser?

Why do I need a NAT rule if the computer uses DHCP?

Makes sense, is their any downside if I set the same DNS in Android and Router?

But Kiwi doesn't open links from PWAs in default Browser, so no solution for me

On Firefox it works if you change the settings, but unfortunately i don't know any Chromium browser with that feature.

It indeed works, but Firefox and derivatives are really slow on my phone :x

In settings there is "open links in apps". If you change this links are indeed open in the default browser