What network are you with? This doesn't match my experience -- I've traveled around 35,000km on trains inside Germany in the last few years, and apart from one 5 minute section just outside of Berlin when going SE dresden, I've pretty much always had a full signal.
YMMV I guess. But I just wanted to say that I don't think this is completely accurate.
The track between Berlin, Hannover, Osnabruck is particularly bad. You get some connectivity near those cities but most of the way my phone is useless.
Telekom is better but they too have issues in the country side. I also encounter this issue professionally with our customers as we sell a SAAS app that our customers have to use on site and a disturbingly large number of our customers have either very poor or no connectivity as they tend to be on the edges of cities or in the middle of nowhere. Which in Germany means forget about 5G and hope for the best with a 3G/4G connection with one or two bars if you are standing next to a window. It's a specifically German problem, other countries in the EU have more modern infrastructure.
YMMV I guess. But I just wanted to say that I don't think this is completely accurate.