Just to be sure: your blog post mentions 24.12.
2024 - did you mean 2025?
Is this the right connection?:
https://bahn.expert/details/NX%2028521/j/20251224-a0049123-9...
The RE5 seems to be operated by National Express, not Deutsche Bahn, right? (but DB InfraGo is most probably responsible for the routing)