In the handset menu, under System > Settings, the Repeater Mode setting has been replaced by Encryption. This setting relates to the new Gigaset DECT repeaters in the EU which use encryption.
These are not available in the UK so you will most likely be using a DECT repeater that does not use encryption. You should therefore turn this setting OFF, to allow you to register the DECT repeater to the base station.
If you are still having trouble registering your repeater to the base, please take a look at our handy guide.