Is setting up the wireless network router necessary?

So we bought this TPlink wifi router and i know a very minimal things about networking.. (which is sad cause i took a networking class). I followed the diagram of how the hardware is set-up. The PC that is connected, there is internet connection. What I want is to have a wifi in our home but this one pc (which is what im using) is connected wired and not wireless since its an old pc and cant detect wifi.

I followed the instructions given on the manual but it failed on the last step. After that I cant access the internet anymore on the pc. Then theres another person who tried setting it up which i didn't know what he did. still not working. What I did was I reset the whole thing to default on the wifi router page.. Now it works! Wireless device can connect and at the same time, i can also browse on my pc (not wireless-ly)

My question is, since I basically reset the whole thing and started from scratch, what's the point of setting up and following the manual? What I only did was connect the 2 routers correctly and thats it..

Also, Any tips on improving wireless connection? Not broadcasting SSID seems like a good idea but.. how can I connect a wireless device if the wifi is not search-able?

Because having two routers in the mix is very, very unusual and can get complicated. A manual may provide help.