You may have already tried this but did you change change the SSID from stock i.e. "DR500GW...." in the Blackvue App ? Again I am sure you are aware if you did this before loading custom firmware those settings would have been lost and it would revert to stock.
Have you tried using the Blackvue app to update the SSID and checking if it is then broadcast (format card first, install in the camera, once updated use the app to update SSID).
The issue with downgrading firmware that I had in the past was that the custom firmware was based on the Russian firmware and so loading English firmware was blocked. I did try it a while back with my camera but failed, I have not looked into it as the custom firmware works very well for me as long as the settings are not too high.