Thnx for your detailed response. Based on your feedback stating that the interfaces needed to be specified, which in my case were missing. In the main status dashboard - Interfaces overview there was only the local loopback interface and no ETH0 or ENS33 interfaces displayed. That led me to the Nethserver developer wiki / forum where a solution was presented by configuring the interface using cmdline:
db networks set eth0 ethernet role green hwaddr xx:yy:zz:aa:bb:cc ipaddr 10.213.0.197 netmask 255.255.255.240 network 10.213.0.192 onboot yes bootproto static
After this i enabled the firewall rebooted nethserver and with shorewall running i was able to access the management interface. Also, in the main status dashboard - Interfaces overview there was now a green ETH0 interface displayed.
Thank you for your help. Is this not a possible bug because I am not he only one with this issue.