Yes, it has the correct SMA connectors for the Spitz AX.
Yes, the 600-6000 MHz support will work with supported cell frequencies. It will also work with the Spitz AX’s GPS.
3-4 dBi gain is not an improvement over the Spitz AX’s included paddle antennas. If you place that antenna beside the Spitz AX, you won’t see any improvement. If, on the other hand, you mount it on top of a vehicle (or building, or whatever to get it up and away from obstructions), you may see an improvement.
I’d missed the ground plane requirement; I was planning to augment the “hook” it comes with so that it could be suction-cup mounted to a window in a hotel etc. but if there’s no likely advantage over the paddles I’ll probably have to pass on it.
Proxicast makes this model instead, so maybe that’s the one I’ll go with: Proxicast 4x4 MIMO Omni Desktop Antenna . It’s just I have to order it from them directly, vs. Amazon (which is sold out) and lose the cheaper/faster shipping (and apparently a lower price overall at that).