After typing the z-wave first five DSK digits the installation fails. I have tried with two different thermostats and both failed
That’s not a lot of information you’re providing.
Z-Wave devices must be included in direct proximity to the Homey (< 10 cm):
My recommendation is to include the thermostats in “unsecure” mode. Instead of the original PIN, just type in the zero 5x.
Thank you! You solved the problem