(One more) Issue with responsive Header-Text

Dear Team,

Im sorry to bother you once more, but theres one more issue on my website that I just cant figure out how to solve:
Im not able to change the text-size of my headertitle on my frontpage (and all the other pages) for mobile devices. I tried out more or less all codes provided for this issue in this forum, but nothing works. I want to make it smaller so the words dont break in the middle.
Heres the link to my website: http://www.hochwasserschutz-unterinntal.at/
Thanks a lot for your help in advance!!
Kind regards

Christina