Can't sideload apps - Lumia 550

I've tried this guide, but t doesn't help. (yes .xap).

Quote:

Head to Settings > Update & Recovery and tap on "For Developers" You will find three options here. Choose the second one which lets you sideload apps from other sources. Next, transfer/copy the appx file of the app to your Phone/SD Card storage. Browse to the file via File Explorer, tap on the file and it will throw an install prompt. If you really want to give it a try, tap Install else tap cancel. If all goes well, you will find the app under the app list under 20 sec.

And this:

Quote:

Head to Settings > Update & Recovery and tap on "For Developers". Under Device Portal, turn on remote diagnostics over USB and LAN connections. You can either turn off Authentication or turn on it. Turning it off eases things a little bit. After turning remote diagnostics On , It will provide an IP Address to connect to. Enter that IP address either in W10 Mobile Edge browser or any PC Browser(depends on where you have appx to install). Make sure your phone is unlocked. After authenticating to device portal page click Apps Section on the Top Left hand Side. Under the Install App division Click Browse to select the Appx/Appxbundle you are about to install. After adding the Appx/Appxbundle just Click "Go" button and that's it.If all goes well you can find your app under app list.

That may be the problem?

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