Same as OP. When i put Pi IP in primary and leave the other 2 to what they were at default Pi isnt working. When i change secondary to the same as primary is errors saying cant duplicate address's. When i leave secondary and third empty or 0.0.0.0 or 1.1.1.1 nothing happens. I get internet but Pi doesn't do anything to block adds or catch really anything.
A friend of mine is of the belief that my PC and IP are using IPv6 and that's whats causing my issues with Pi. But he doesn't have any experience with Pi and IPv6 so he can't help. As you can see in my post i made the other day after showing screenshots of my router and ipconfig the thread died when it was brought to the attention to people that i was using IPv6. Scroll torwards the bottom. but read the last message DeHakkelaar wrote