DFS is a 5GHz feature. It could not work with 2.4GHz.
If you mean ‘2.4GHz is working great, every time’, okay.

I can’t see this behavior with my Slate AX. I will test again, later.
I can imagine you’re testing near an airport?

Take a look at Dynamic frequency selection - Wikipedia … especially the part

During this phase, the access point is in a passive state scanning for radar signals. This commonly takes 1-2 minutes, but could take up to 10 minutes.

Did you tested for more than 10 minutes?

I have no insight in how exactly the Slate AX is working. Maybe there are also some logs, that shows if/why the connection is pending.