So here was the solution for me coming from synology assistance (Malek was super nice to help me):
1/ On the NAS, in control panel, “external access”, tab DDNS : create a synology address… it will be something like that: <name>.synology.me
2/ On the NAS, Set a fixed ip address on the local network to the nas : to do that, go to control panel/network/tab:network interface and modify the LAN line “manual ip config” the line IP address with the number you want (for me 192.168.1.24)
3/ on your box, open ports 5001 (connect to it through its local address and modify its NAT configuration. open a port for your nas in TCP port 5001
so here, the nas is accessible fro the outside on its caldav
4/ on the android DAV5X, open it, add a account, select URL and there add put the URL: https://<name>.synology.me:5001/caldav/<your_id>/aiioyb/
and enter your <your_id> and your password (the one to connect to your nas, at the id session).
Recommended Calendar App?
I just setup a Kolab Now Account and installed davdroid to sync the calendar.
However, until now I had no need for a calendar on my mobil devices, therefor I wanted to ask, if there is a calendar app that you could recommend, because it works well with davdroid.
Kind Regards and a Happy New Year
Thank you for the quick answer, in this case I will go with S Planner as well.