Troubleshoot Android strange behavior

[Skinoku]

Yesterday I checked the UPNP ports mapped on my router and so I discovered that my galaxy s5 running 4.4.2 maps at every connection the port 1024 calling it SEC_NAT_192.168.X.X:1024 with TCP protocol.
A scan with ESET revealed no malware and the phone works just great but I was wondering what might be...
Thank in advance!

 

[LabZero]

The ports from 1024 to 65535 can be used by non-common services or applications.

This does not necessarily mean a problem but you can install Adguard free and enable the firewall, under settings.
So you can check for any suspicious (or not) network communications.

Adguard for Android Overview - the world's most advanced ad blocker

 

[Balrog]

Also i suggest to run Zemana Antimalware for Android. There's plenty of android ad based apps and games.