I would recommend to set the netgear router to MODEM only and let the Deco have the PPPOE username and password.
Better turn of the netgear WiFi as well, if it's got any.

I don't know if my experiences apply, but my ISP (Softbank in Japan) provides a router with the IPoE information on it. I tried to bypass this and use PPPoE set to the modem first, then to the Deco, but both didn't handle it well. I finally managed to get up to speed by connecting the ISP's router to the modem (with default settings), and then connecting the Deco to the ISP router. This allows me to use router mode on the Deco to gain the features offered, and I've had no problems yet.

I would suggest the same as tamer.wilson, but if that doesn't work, try the same setup as mine with the Deco connected as Dynamic IP to the Netgear as modem/router, and then don't connect anything else to the Netgear.

If all else fails, you could always use Deco in AP mode, but you'd lose the QoS and parental control features.