New install, all well except for Pro6 Outdoor

I just finished installing 3 Pro6’s and 1 Outdoor. Only one AP is direct, rest is mesh. All the indoor APs are fine, but the outdoor one will not connect to the mesh. If I connect it via cable, it will eventually build a connection to another of the indoor APs, but choose Ch 100 on 5GHz.

I was advised by a friend to set the same Channel for all APs (36) and initially I had issues with choosing that in the Outdoor, but eventually it became an option, so I changed it, but then unable to connect. I found a thread that mentions similar issues: 5Ghz channel “locked” on AP6 Pro Outdoor! - Access Points - Alta Labs5Ghz channel “locked” on AP6 Pro Outdoor! - Access Points - Alta Labs

I am sure I will be able to connect if I connect a cable again, but ideally I want a Mesh for easier install.

All APs are running latest 2.4e FW.

For testing, I reset the Outdoor AP, put an indoor AP very close to it to make them mesh, but had no luck connecting them.

Any thoughts on this?