Wekan/Docker on Synology access problem from the internet


Hello, helpful community!

Today, I installed Wekan on my Synology DS218+ with Docker. I managed to get it running using the local URL and local port “LPORT” => http://192.168.ABC.XYZ:LPORT/wekan

I also want it to be accessible from the internet, say at https://ROOT_URL:WPORT/wekan or simpler at https://ROOT_URL/wekan
The Synology can be reached via https://ROOT_URL:SSHPORT/
I set WPORT to be forwarded on my router.

Unfortunately, I cannot reach Wekan from the outside. What am I missing?
Your help is much appreciated.

Kind regards,