I try to register a Gigaset E560HX on a Fiber Box X6 (which has DECT feature), but it doesn't work because the telephone fails to ask for the 0000 pin
Is the Gigaset E560HX compatible with the Fiber Box X6?
If yes, any solution/suggestion?
As this forum is mainly used by German customers I think that no one can help you with your Fiber Box X6 as this is an uncommon device in Germany.
I know routers in Germany that accept registering device without the need to enter the PIN code, however I don't know how it is handled by your router.
Usually you have to start the dect registering process on the router and then also on the handset.