Hm, TCP-6556 is FROM the librenms, so you don’t need to open ports on the container side, you would need to check firewall and allow TCP-6556 on the device with the agent.
I came across this: Hi everyone! Question about LibreNMS and the check_mk agent - LibreNMS Community
I would do the following:
- Ensure firewall allow on the device with agent
- In librenms, under the device with agent, Settings > Modules > Enable Unix Agent (Left of page)
- You have to manually allow the apps/modules that you want to monitor on. Application Docs / Agent
- On same device page, 3 dots (beside Settings/Gear Icon) , click on Capture, under Discovery, click Run. Check for check_mk logs (screenshot: LibreNMS and Raspberry Pi - #3 by jasonloong)
- Do same as above for Poller, and check logs on the check_mk/unix agent
If that still does not work
- You have to manually check the scripts provided and make sure they point to correct bin PATH if non-standard Linux like FreeBSD
- If Windows: make sure firewall is allowed
Hope that helps 
Let me know if that worked out!