Don’t get it. I give up.
Balena is installed on a normal raspberry pi, why is there not simply a cron option to start a reboot.
No idea where I should place that and why it is different from other linux methods but I thank you very much for your answer. I am pretty sure its an easy stuff for someone having enough knowledge and that the problem is me not having that. 
The device is dissapearing from airplay from time to time and a reboot often solved the issue so I thought I could easily add a line to a crontab somewhere and its done.
The method used on balena is …“different” …