My new Samsung tab A (Android 5.1.1) keep falling to sleep. Pinging from router get 'time out'

I try to use this new tab A as a FTPserver, the Foscam (IP cam) is the ftp client. Before that I have a Samsung tab 4 (kitkat 4.4) and it ran fine (same router). Before that I had
a Toshiba window 7 as the ftpserve, no problem either. Now with this new tab A, it keep going to sleep. I was a router person before i retire, i keep checking on the router side, get no reponse from this tab A, when ping.

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