Look app doesn't always work when connected to WiFi in the place of the DVR
Take your phone elsewhere and connect to another WiFi source
In ksa not everyone has data allowance so hence I understand why you want to do it via the WiFi
But in this case from the DVR location I would concentrate in getting it to work via local IP not ddns unless your out and about
I don't know why the app does this , but i have seen this occur on many iphones
Am I making sense , not to many lol
So first steps you have done , local.PC can see the webserver and Samsung can see your DVR
Your just stuck with the app
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Earlier on it would work because you had dhcp and hadn't tried via the app then we went onto ddns which also might not work because via WiFi it doesn't always depending on setup
So if your at home with DVR and connected to WiFi , set all back to IP address internally has to work
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Also he doesn't have restrictions because Samsung can see the device , and he can still make it work on the internal network