Can the Deco in router mode act as the ISP router/modem.

I have sky and want to ditch their supplied router/modem. 

The reason is that when the Deco is behind the ISP router in Router mode, I can't access the skyQ recordings on the sky app. I have to put Deco into AP mode.

The only way I can get around this is when  Deco is in Router mode I have to sign onto the sky network to access recordings via the sky app. Don't want to keep changing wifi networks. 

Is there a way around this keeping Deco in Router mode. Is this maybe a double NAT issue.

Many thanks.