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

@holgerlxxiii Oh, I found this late.
I was able to add my CalDav account and see the calendar on my server but I don't think it syncs (either way phone-computer nor computer-phone)

I didn't test it properly, I should delete other calendar accounts and try how it works