Hi Simon,
i never had troubles that the Toniebox would not recognize the tag if it works with my NXP smartphone app.
But remember, the Toniebox puts the tag in privacy mode when you place it at the box. So the smartphone app will no longer recognize the tag until you disable the privacy mode (knock method).
Greetings and good luck with your custom tag.
Hello,
Thanks for your feedbacks. It seems my soldering wasn’t good enough. After several attempts, I finally created one that was immediately recognized by both the smartphone and the Toniebox.
@Robert: Good point - but the first tag was never recognized by the toniebox. So no privacy mode, just another broken chip 
So, I ordered a thinner soldering tip and a finer soldering iron. I’m now waiting for the package to make several more tags.