If you can get the Sky channels list/epg (without which any digibox won't work!), that's the default transponder. It should also be the strongest signal, within reception area, ie you should get the epg even if nothing else.
Grundig brand digibox is well obsolete, but should still boot up ok if working. Otherwise, you may have to wait a while for new software this make. If it's not working, that would mean knowing when the new software will be transmitted, then plugging into known working dish and "hoping" the problem's just corrupt software!
I don't know how to find out when the next Grundig software will be transmitted (Sky customer services are famous for not knwing things like this!). Does anyone?
If box seems functional, ie if it responds to remote control and shows menus on tv screen, you can try a manual install, or even "other channels" to check it's getting a signal.
Yes Iceman, I agree forced update can kill a box, it's not something you should do without a good reason (usually, box is already dead!).