Ok, just continued my analysis.
I have 2 ports exposed. 1 is currently accessible from the internet in this case the UCP services. I used that working services to test my portforwarding in my router. In other words, both forwarding rules for the different ports end up at the UCP http port. Conclusion → Modem is not the problem.
Turned of my FreePBX firewall… and admin panel is working again. How can I restore this within the firewall of FreepBX?