Honestly, I can’t understand the problem.
Are UDP packets reaching NethServer wan?
Run:
tcpdump -nnpi <red_if> port 1194
and try to establish the vpn. If you see no packets, you must reconfigure the fritzbox until it works.
If you see packets, they might be dropped by the Nethserver itself. You may see them in firewall.log.
Try again disabling NethServer firewall: shorewall clear
Seeing firewall.log we could find a way to reconfigure the firewall.