Radio communications were still a new technology, one that was relatively untested on the battlefield. And they were also a form of communication that was notorious for having spotty connectivity when within mountainous regions.
Hence why most military units also typically carried a satellite phone, and at least one other form of communication during Bruno's past life. Thus, it was understandable as they were rolling through the mountains that communications that were designed to be relayed between the front lines of the alpine theater and Berlin would suddenly go dark at a critical moment of peace negotiations.
Or, at the very least, it was plausible enough that Bruno had an excuse for outright disobedience and insubordination, two things that in this day and age would get you the firing squad if severe enough. Why had Bruno feigned these communication issues, and disobeyed his direct orders, which came from the top of the German Military and the Kaiser himself?