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.
Anyway, in this thread toggling flight-mode reactivated Wi-Fi in 5 GHz. That is because Windows 10 Mobile restricts the usable Wi-Fi channels depending on the country of your SIM. If the connection to the SIM is lost unintentionally, Windows 10 Mobile falls back to 2.4 GHz only, channels 1 to 11. Therefore, if the flight-mode helps in your situation, you are actually facing a different cause: The SIM gets lost internally. In that case, the app Field Medic should help to determine why this happens. However, I do not know how to use/read that app. Anyone?
Anyway, in this thread toggling flight-mode did not help and the whole phone had to be restarted. What is your situation?