I feel you. We have stellar coverage pretty much everywhere in NL. Heck, I was recently in a work video meeting in the car, not a single drop. The route included part of this:

https://en.wikipedia.org/wiki/Afsluitdijk

Yet, when we visit family in Germany, five minutes after crossing the border we are in a cellular dead zone.

Interesting to see you have a different experience. I'm not sure I would call it stellar. On the train route between Den Haag and Amsterdam, one of the busiest routes in the country presumably, reception is constantly dropping out. I'd love to be able to work on the train, but it's completely impossible if you need a network connection for anything.

Perhaps the route being so busy is the cause of the connectivity issues, but it's still baffling to me how bad it is, given that the amount of mobile devices trying to connect must be very predictable.

Germany is a developing country when it comes to broadband let alone wireless internet.

The short version is that the chancellor we had in the 1990s didn't like how the public broadcasting channels were talking about his failures and wanted to push the development of private broadcasters (who being beholden to financial interests rather than objective news coverage mostly spoke favorably of him) by prioritizing cable television over fiber. A surprising number of things came downstream from that pivotal decision, e.g. the completely braindead way we sold frequency bands (which resulted in some literally remaining unused because there were initially no requirements to actually do anything with them).