Port forward explanation needed [problem ended]

I'm on LEDE 17.01.4, freshly reset acs1900v2, with basic network configuration via graphics interface. Nothing installed, nothing modified (afaik). I want to set up port forwarding from wan to lan, which does not work as expected.

It seems that if a port to forward is not "high enough", it does not work, eg: a port 15000 (decimal), the connection gets stalled (no reject, no drop). It port is eg. 55555, it gets forwarded.

I tested it on a few numbers, dont know it is a strict border somwhere. I cannot find any anomaly in firewall rules (iptable-save |grep my_rules). In the firewall status the incoming packets might be last traced in the prerouting.

What can be the case behind? Is there any limitation in port numbers to forward?! How to investigate what happens?

Ports lower than 1024 are considered "privileged" in UNIX environments, but LEDE does not make any restriction about them with respect to forwarding.

I would bet that your ISP is blocking those connections.

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Thank you for notification, there was no ISP role, I tested outside connections from my own perimeter.
Sorry to say, the phenomena vanished. After one week struggle suddenly cannot reporoduce the case, everithing works. The bad is that cannot explain how and why.

If the problem occurs for you again, running this tool may help understand why: