In my opinion that won’t work directly because you can’t send a calculation to a sensor.
I suggest you take a look at @prohand’s tutorial on automating the washing machine.
Actually you’ll need to create 2 features for your thermostat temperature: 1 for getting the temp (sensor), 1 to do the calculations (actuator).
In the tutorial it’s called sender (sensor) and listener (actuator).
EDIT: after rereading your post, it seems to work because you get 28 after the +1.
If you send the value of your button to yourself via Telegram, what number does it send back? (it won’t return the z2m result but a numeric value normally)
If there’s nothing then you should create a new topic to add the specs of the Aqara W100 to Gladys.