Hosted Site unreachable outside Network (only HTTPS)

[SydonX3]

EDIT: I've managed to track it down to my router that seems unwilling to forward port 443 at all. Since I can't delete the thread, I'll leave it for anyone that is interested in a port-forwarding issue or debugging of similarissues.

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I managed to set up Reverse Proxy to one of my Docker container. (It runs Strapi, which only supports HTTP):

This works fine from within the network, but it fails from the outside (Page cannot be reached. Telnet timeout).
The portforwarding of the router looks correct to me:

I also set up the connection over HTTP. This works from inside and outside the network (Page loads without SSL. Telnet connects):

The tests were all made while the Firewall was disabled.

I'm currently out of ideas, if anyone has a hint for possible causes, please let me know.

-- post merged: 29. Nov 2021 --

My current thoughts are:

• Since I can connect from within the network, the SSL setup should be fine and the Docker container runs fine.
• Since I can connect from outside the network using HTTP, the Dynamic DNS (in use for years) to the router works fine.
• I don't know whether the port forwarding works.
• The Reverse Proxy should work since it works from within the network, but I don't trust it.

-- post merged: 29. Nov 2021 --

Update: Using Webstation to host a PHP page without any Reverse Proxy involved results in the same behavior. Therefore it should be a problem between the Router and the DSM?