Thanks for the links! I went to take a look and some users are reporting the same problem. If I understand correctly, for some, the problem resolved itself “by itself” given a little time.
I still called 1st level support, who told me after a few minutes of “tests” on the Swisscom infrastructure, that "everything is fine on our side, and I see that you are well connected in 5G because It says LTE.” To which I replied that 5G is not LTE and that in the Booster interface it is indicated that the 5G network is “disconnected”.
Unfortunately, nothing happened, my interlocutor persisted in his response. I still insisted by saying that I had no speed gain, that the flow was the same with or without Booster. To which I was told to “do tests at three different times of the day, TV off, WiFi off and call back”.
In short, I basically haven’t made any progress thanks to the support regarding my initial problem…
Apparently @Roger G from Swisscom seems to know the subject better based on the discussions in the Swiss-German posts.