Devolo thermostat and "unknown" values

I bought Devolo thermostats and they appear in the Home Assistant. I can adjust the temperature and the actual temperature is displayed (Devolo can send it to Home Assistant). The only problem is that the text “unknown” appears in front of the set temperature.

I read instructions on how to fix this by using “custom ui”:

Unfortunately, I did not know how to do this.

Would anyone be able to advise on how to do this? What does that “state-card-custom-ui.html” include?

Or is there any other simple way to fix that “unknown” problem?

Thank you!