I think Scott meant: “There is no special/new firmware update (just) for Windows Insiders. Firmware updates are not part of the pre-release rings (Source).” Consequently, you three are all correct.
Ok, no problem :)
Toggling flight-mode NOT reactivated Wi-Fi in 5 GHz for me. I need to restart my Lumia 950 XL Dual SIM. The loss of signal is very rapid (some minutes to some hours), this solution is not acceptable for me. Also, I confirm that my router is configured to use channel 1 only so it's seems ok with your details about how to operate Windows 10 Mobile.
Considering having no problem if I'm using a WiFi in 2,4GHz, it's clairly looks like a problem with drivers/firmware or with Windows 10 Mobile. But on PC, I have no problem with a WiFi in 5GHz so it's maybe more a specific drivers/firmware bug for Lumia 950 and 950 XL. Maybe only with Dual SIM models.
If this is a problem specific to Mobile, shame Microsoft did not implement a toggle for disable the 5GHz band (on a mobile, speed of the 5GHz band is appreciable but not necessary all the time as on a PC).
I'll do some tests with Field Medic.