Then that's very strange. I did this probably about 6 years ago on a WNDR4800 or WNDR3700 or whatever NetGear routers were out at the time. Worked perfectly for me.
I'm trying again right now, I will update you as soon as possible.
Anyway thanks for your help.
Just spitballing here... Alternatively you can use the WiFi as your client on the WAN network and use SNAT to rewrite the LAN address from the secondary NAT to the primary LAN subnet for the NAS. For this to work I'm sure you'd need a static route on the primary router pointing to the secondary router's IP and some form of forwarding/DNAT.
I tried again, but i have the same issue.
When i create the bridge the wifi interface stops working and i see this aside the configured wifi client in wireless section.
I also tried to reboot the wifi interface and the router but nothing changed, only after removing the bridge it connects again.