Then someone is on this forum 🙂 Thank you for your answer !
This afternoon, after my last post, after all these days, I solved.
An oversight as trivial as it is difficult to identify: it was my port assignment error.
As you know Resilio uses port 28888, which I promptly inserted in the Fritz! Box router rules on the internal IP of the Nas Synology.
The fourth group of digit that make up the IP of the NAS is "100" and in writing the rule in the Fritz!Box router, for too much confidence, too much speed and little head, I wrote 101. Damn me ! 😈 That's all.