The IP set during a clean installation is changed after the first reboot

What kind of installation did you do? Interactive?

Could you paste

 /root/anaconda-ks.cfg

There’s an important notice about static IP assignment from Anaconda

Under the Network > General section, only the interfaces marked as Automatically connect to this network when it is available are enabled at boot in the installed system. For more info, refer to RHEL 7 installation guide.

http://docs.nethserver.org/en/v7/installation.html#interactive-and-manual-modes