I have an update on my case.
Wingo support tried to reset my esim, first with a light reset, it didn’t solve the issue. Then with a more profound reset (I have lost the signal for 5 min then I had also to setup from scratch the comm box). Both of the resets didn’t solve the issue. In the meantime I have also request an eSIM of replacement, and I have activated it directly under the iOS 18.2. The issue disappeared for 12 hours then again, speed cap at 100/8 Mbps.
I have noticed a new thing though: if I restart the iPhone, the service comes back again at full speed.
I’m starting to think that it’s a bug of the eSIM or of the Qualcomm modem inside the iPhone. Last chance: I have ordered a new physical classic SIM card and I’m waiting to receive that via the post.
Next week the tests will be continuing.
If nothing is solved I think I will contact Apple. It’s really a mystery.