thank you for the clarification
So I tried to start sync on my Android device and I indicated my ddns address like https://myname.synology.me:5006/joplin/ but after several hours nothing seems to have progressed: 6/6 sync objects/total. While I actually have 149200 objects in total.
I'm connected to my local WiFi, I read that could be the problem.
I'm going to try with my local fixed IP in http...
I will continue this discussion on this separate topic...