Plug your phone in USB to a computer, ideally running Windows.
If you see a USB device plunging/unplugging about every 3 minutes, your device is in EDL mode.
Some help about this : Data transmission per USB to PC not working (Win10 doesn't recognise FP3) - #3 by smu44
If you see the symptoms for EDL mode, you can try to flash over the Gigaset firmware : /e/ installé avec easy installer sur GS290... le téléphone ne démarre plus - #17 by smu44