[inquisitor]

"Nur erreichbar" means "only reachable" which usually only shows up if your SIM card has been blocked for all outgoing connections due to the verification letter having returned. I guess your hotel didn't accept the letter or returned it for whatever reason. Perhaps they thought it was junk mail because guests usually won't receive mail from a phone company.
Anyway, you will need to contact customer care and tell them that you are a foreigner and that you used your hotel's address for registration. If you also provide your address in the UK, they will reactivate your SIM.
Lidl do not have any restrictions on the allowed hardware, so using your Lidl SIM in an iPad was absolutely legal.