802.11b is enabled on the access point. I verified it by setting a windows tablet to only use b and it connected; then checking the unifi controller, it shows the connection as b.
I have essentially done what you talked about and pulled an old unifi AP and added it; it works, so there is something about old vs new AP that all of the settings I have tried make no difference on the new AP with the Pronto.
I am just trying to not have 2 AP at the same location, one for the pronto and one for everything else.