@holgerlxxiii

I found it out by my self.
Take the CalDAV adress which Synology recommends for Thunderbird (not macOS / iOS):
Synolgoy Calendar:

Put the the server address, user and password in the dialog above (Ubuntu touch: Settings -> Accounts)

@AppLee @Bolly
I see no easy way to do this.
However I apreciate your replies. Thank you!

Can this be done? Thank you

@holgerlxxiii

I found it out by my self.
Take the CalDAV adress which Synology recommends for Thunderbird (not macOS / iOS):
Synolgoy Calendar:

Put the the server address, user and password in the dialog above (Ubuntu touch: Settings -> Accounts)

Hello, did someone get this to work?
What I have to fill in here?

I have access to my CalDAV on DMS with: https://<test>.synology.me:5001/calendar/

Apparently UT removes "https://" automatically and puts the username before it.