1st interop unlock ur device...Check This link - http://forum.xda-developers.com/wind...terop-t3135326
2nd now download the file...
Interop unlock archive.zip
Download Here - Permanent interop Archive.zip
U will need a pc/laptop to do this hack.(i hope u have one or managed one )
Now extract the zip file..u will find 3 files ..
Now install 7zip in ur pc. No need to open the software.
Now connect ur interop unlocked phone with usb and deploy vcreg1.2 in ur phone with any deployer.
No need to install that again if u alredy installed the app earlier.
Now open the vcreg 1.2 app in phone...click on( ••• ) ...then press template .. Now mark the live full file system access.then press apply...
Ok it will show complete...
Now Restart ur phone and connect it to ur pc..
*******It will take some time to show/detect the phone.(keep calm)
(again keep calm.don't be mad if it took so long ) ********
Ok when ur pc detect the phone.go to (Ur phone>Phone memory>windows>packages>registry files) folder..now u will see many registry files.
Warning:: don't play with those files.
Now scroll down and copy the (software.reg ) file from the folder to ur pc.
Now the main work begins.
Right click on the software reg and open it with 7z file manager(not 7zip)
Now u will see (software) file in 7zip fm...again right click on it and select edit...
A notepad tab will open with some text.scroll down and goto the end of the text.
Then open registry entries.txt file i provided with zip.and copy the whole text from reg entries.txt.
And paste the texts under the software texts.
Now press ctrl+s.
Press ok...again ok...
Now copy the modified software.reg and paste to( Ur phone>Phone memory>windows>packages>registry files)
Press yes if u see any warning.
Done. Enjoy the unlock device forever.
Here is a Oemsettings procedure..thanks to @rickk182440
Hey , that's the way to have Interop forever on windows phone 8.1 and also the windows 10 mobile , get tested , it works !, even after upgrading to Windows 10 mobile and make hardreset , it continues with the Interop done, and a few more alterations . Only add these records in OEMSettings.reg and format that will be with Interop
Oemsettings reg entry.txt