We can start to test NethServer on the newly available CentOS 7.5.1804 ISO. Just follow this procedure:
@giacomo has backed an ISO!
Download NethServer 7.5.1804 Alpha 2 ISO (993MB) temporary download link 
As alternative, install NethServer on CentOS 7.5.1804 Minimal:
(1) Download and install CentOS-7-x86_64-Minimal-1804.iso from CentOS mirrors
(2) Install nethserver-release:
yum install http://packages.nethserver.org/nethserver/7.5.1804/updates/x86_64/Packages/nethserver-release-7-5.0.2.g2ec7a1c.ns7.noarch.rpm
(3) Require a fixed NethServer release number
echo 7.5.1804 > /etc/yum/vars/nsrelease
(4) Enable nethserver-testing repo, by editing /etc/yum.repos.d/NethServer.repo. Set enabled=1.
(5) Run nethserver-install as usual
(6) Connect to Server Manager
The development of 7.5.1804 is tracked here: