Dns is really always the problem? LOL

I solved a problem today that was very crazy. First things first, I have a raspberry running a pi-hole and its the default primary dns to all devices in the network(but its not on all the time, only when I turn on my pc, that was not the case today, it was off).

But I was trying to use a specific app on my smart tv(I use wireless by default), the problem was that all other apps were working fine, but this specific app was not, so I tried to check if using the ethernet cable it would work, and even using it the app didnt work.

So I put the cable at the ISP bridge router, the network is:

bridge router <-> my router <-> LAN 

And it worked, but the tv was receiving a public ip and I didnt want my tv exposed, so I thought "Could the raspberry dns server that is not running break the system?", and then I changed the dns from:

primary -> 192.168.0.105 secondary -> 1.1.1.1 

To:

primary -> 1.1.1.1 secondary -> 1.0.0.1 

And it worked! Idk if I could explane in a good way what really happened but the central question that I have is:

The dns order(primary and secondary) works like -> use primary but if not working go to secondary or more like round robin, distributing the traffic?

P.s: The more I study network I realize that I know nothing LMAO