Hey there, new to Home Assistant and was experiencing the same issue. I put this on a similar thread but I thought I would respond here too since this is more recent.
There is a init .py file in the core/components/climate folder that defines the max temp for the device. I found mine here:
/usr/local/share/homeassistant/lib/python3.8/site-packages/homeassistant/components/climate/__init__.py
Find the appropriate line:
DEFAULT_MIN_TEMP = 7
DEFAULT_MAX_TEMP = 35
Define it as you wish (in Celsius, obviously), restart, and the thermostat card should now use the defined min and max.
UPDATE: Also can be done on an entity basis through the UI, much easier:
Configuration > Customization > Select Entity and change min and/or max temp value.
I’m sure there’s a way to do this through YAML too but these both work for me.