AC1900 Dual Band extender mesh networking

So I feel like I tried everything. I’m in a concrete house trying to get signal strength to the third floor. There is a long tall staircase going up to the third floor, a landing and then the full hallway around a corner.

So I had 1 Netgear extender, Mesh Ac1200 model # EX6150v2. It was all set up and good to go. Then I wanted to add a EX6400-100NAS.

And it refused to connect. I even unplugged the first extender and took the second one down to the router, factory reset the extender, started over setting it up, bringing it back up stairs and plugging them both into the spots I want.

I also tried to leave the first extender with the One Name setting and have the second extender have it’s own name, but no luck. Superrrr stuck here