I see! just use https://ip-or-fqdn/zm without specifying the port :980
e.g. https://192.168.1.2/zm
Just to test it I followed the guide but with some small changes:
Install zoneminder on NethServer 6.8 (64-bit)
yum -y install nethserver-mysql
yum -y install http://zmrepo.zoneminder.com/el/6/x86_64/zmrepo-6-7.el6.noarch.rpm
yum -y install zoneminder
eorepo centos-{base,updates} nethserver-{base,updates} nethforge
mysql -e "create database zm"
mysql -e "grant all on zm.* to 'zmuser' identified by 'password'" # replace 'password' with your choosen one
mysql < /usr/share/zoneminder/db/zm_create.sql
echo "/etc/zm.conf" > /etc/backup-config.d/zoneminder.include
sed -i s/zmpass/password/ /etc/zm.conf # replace 'password' with the one you set before
service zoneminder start
service httpd restart