I also found another app in Jolla Store which works well with Tasmota/Sonoff; that app is 'd1webclient' by jsm.

With a few minor changes made in the UI, it is very usable. With this app, you can combine all functions to its 'main page', so no need to see the full Tasmota UI every time, quite handy I thought. Functions can be added to the cover; example: I can turn off my desk lamp from the cover - very handy indeed if you have one particular function you use regularly.