Developer Mode Issue solutions(maybe)

So I had the same issue as you met.  And I have gotten it back now.

 https://developer.oculus.com/manage/verify

It worked for me. Oculus requires payment verification for the developers. So I filled them in and got my mode back.

There is the other way to solve it :
Oculus Developer Hub(or you have it before)

I heard someone succeeded by using ODH to get it back.  Reconnect your device from "Set up new device". And this requires the Bluetooth function on your PC，follow the guide to set it up then you can activate it from PC