Time Window for Wake On WAN

With iPad Pro controlling Win 7 Pro PC I have no problems with connecting on my LAN.

I am also able to connect over an external internet connection (WAN)

If my PC is switched ON it is always available to the iPad Pro on the WAN.

I can then Suspend or Shut Down the PC.

However I then only have a time window of about 5 minutes for Wake Up to work.

If I leave it any longer than 5 mins Wake Up will not work.

Has anyone any suggestions for this problem ?