Warning: what's written below is something I've done on my own phone, but others have suggested it's a bad idea (see following comment from UniSuperBox). For me it's the easiest way to directly have a copy of a calendar on both my desktop computer and my phone, and thus I share it as a possible solution. But, I acknowledge that others with more experience advise against it. And, I also acknowledge that some people hate command-line programs, so what I suggest below may simply not be of interest.
Command-line Linux programs work on Ubuntu Touch. So, a possible solution would be to install something like calcurse (it's available as a package) on both your Ubuntu phone and (presumably) Ubuntu desktop. Then simply "sync" by replacing the "todo" and "apts" files, via a file manager, as needed (IE, whenever one of these files has been altered on either the phone or desktop). These files are located in the following directory of both the phone and the desktop: ~/.calcurse
To install it, run the following from your desktop while your phone is plugged in via USB:
mark@debian:~$ adb shell
* daemon not running; starting now at tcp:5037
* daemon started successfully
phablet@ubuntu-phablet:~$ sudo mount -o remount,rw /
[sudo] password for phablet:
phablet@ubuntu-phablet:~$ sudo apt update
phablet@ubuntu-phablet:~$ sudo apt install calcurse
phablet@ubuntu-phablet:~$ sudo mount -o ro,remount /
And that should install it. The last command is to leave the system as read only (ro), which I gather is important. I obtained the commands to allow for usage of APT from the description on installing Anbox (see here: https://docs.ubports.com/en/latest/userguide/dailyuse/anbox.html)
If you don't mind command-line programs, the following are ones that I find useful.
Midnight Commander (known as "mc") is a recommended command-line file manager. Also, "locate", which helps with finding files after running "sudo updatedb", is also a useful command-line program.