Yes, there can be penalties for using an unapproved mobile signal booster in Germany[3]. The Federal Network Agency (Bundesnetzagentur) regulates mobile signal boosters, and devices need their approval[3]. Non-compliance can lead to fines and confiscation of the equipment[3]. Using unauthorized devices can disrupt public telecommunications, which is a criminal offense, because these devices transmit on protected frequencies without a license[1].
Using unregistered signal boosters is illegal because they use a spectrum they haven't paid for[2]. They interfere with airwaves, distorting mobile signals for other customers nearby[2]. Additionally, these devices can cause mobile handsets to transmit higher power, leading to higher battery consumption and poor network performance[2].
Citations:[1] https://www.reddit.com/r/AskAGerman/comments/191j9b2/is_installation_and_using_a_mobile_signal_booster/
[2] https://digitalpakistan.pk/why-your-mobile-signals-drop/
[3] https://www.mobilesignalguru.com/blog/legal-requirements-and-regulations-for-mobile-signal-boosters/
[4] https://mobilesignalsolutions.co.uk/legal-mobile-signal-boosters/
[5] https://mysignalboosters.com/de/faqs/
[6] https://ec.europa.eu/docsroom/documents/7718/attachments/1/translations/en/renditions/native
[7] https://nextivityinc.com/blog/mcmc-fines-up-to-rm500000-for-using-unauthorised-mobile-signal-boosters-and-repeaters/
[8] https://www.telcoantennas.com.au/blog/our-blog/your-questions-answered.-1-legal-vs-illegal-repeat/