Weird issue with RTN66U: could use some help

[SanPe]

Hi,

I have an asus router N66U, with the latest Merlin build (380.68-4). This router is behind my ISP box, and it's in DMZ. Connected to the router there's a Synology Nas (which is hosting a reverse proxy), and various other devices.

Everything is working great since 3 years. But last week, this thing started:
- Wifi disappeared, like if it was off;
- Internet was unreachable;
- Router GUI was unreachable;
- Lan was OK.

I rebooted the router, Internet was reachable for 2 minutes, then everything crashed (wifi, internet, gui). I flashed again the firmware, thinking that a good old reset could solve this, and everything got back to normal.
And today, exact same issue!

I tried this :
If I unplug the Ethernet cable between ISP box and router, Wifi goes back on, GUI is reachable.
If I unplug the Nas, and plug the Ethernet between ISP box and router, GUI and Wan are reachable, and Wifi is on. If both are pluged-in => router crashes.

I checked the log of the router, and saw nothing special...
I didn't change my settings of router, nas and ISP box. It started to hit the fan like that, no signs of anything, nothing.

Can someone help me? I don't really know where to look to understand what's going on.