Should I or you ever need to delete all registered devices for WiFi access to the Mitsubishi Outlander PHEV, I found a solution on some English forum (I hope, it is OK to copy it here):

WiFi Reset Procedure:

1. Get in the car and Fully close the Drivers door.
2. IMPORTANT - All steps below must be performed within a total of 30 seconds.
3. Without pressing the brake pedal, press the power button once so that it glows orange (ACC mode).
4. Turn on the Hazard lights.
5. Now quickly alternately press and release the LOCK and UNLOCK buttons on the key fob 5 times, i.e. 10 presses in total. This must be done within 10 seconds of pressing the Power button ON as in (1) above.
6. Listen for one BEEP, followed by 0, 1 or 2 more beeps.
7. Now quickly alternately press the LOCK and UNLOCK buttons on the key fob another 10 times each within 10 seconds, i.e. another 20 presses.
8. Listen for one BEEP, followed by 0 more beeps.

If you only hear a single BEEP, with no following BEEPs, you have successfully reset the WiFi. You can now register 2 new phones with the car again, using the standard procedure (but see Note below). If you hear more BEEPs after the first long BEEP, then its not worked and you should try again following the procedure correctly step by step and within the timings stated.

Switch off the Power button and Hazard lights.

Note: If you reset the WiFi, before you attempt to connect your phone to the car again, you should first remove the Mitsubishi APP from your phone completely.
Once the APP is removed you can then connect your phone with the car WiFi first and only then, after successfully connecting your phone (not the APP) to the car WiFi, should you download and install the APP again. Then of course you must go through the APP procedure.

On some Android phones (like mine) after you first connect to the car WiFi with your phone, a warning will pop up on your phone after about 10 or 15 seconds stating there is no internet connection with this WiFi and it will ask you if you still want to connect to this particular WiFi now and in the future. You MUST wait for this pop up on your phone and select Yes (of course!).

I addition, some German forum covers that as well.