R7000 Internet Drops when shutting down WOL PC

Hello,
I have a strange issue. I flashed my Netgear R7000 with dd-wrt (DD-WRT
"Kong Mod" for NETGEAR R7000 2016-09-09) and configured WOL for one
Windows 10 PC.
Everything works perfectly until I shut down that WOL enabled Windows 10
PC. Exactly 15 minutes later the R7000 loses its internet connection
for a couple of minutes and that continues every 30 minutes until I turn
on that Windows 10 PC again. Local LAN keeps connected during the
Internet drops.
I thought the network card could be the problem so I tried another network card but no luck. Exactly the same issue.
Does anyone know how to solve this? I don't want to go back to the stock firmware but if I can't solve this I will have to.
Thanks in advance.