@samir_sabri 

You can setup the Deco WAN settings to be a PPPoE authenticator, meaning you can set your ADSL PPPoE username and password on the Deco for it to get the IP address and thus use the rotuer in routed mode to get access to all of its features.  It cannot however replace the modem.  If your sagecom is a dual purpose gateway (modem and router) you would want to put it in bridged mode.  This disables the router features of the sagecome but keeps the modem feature active.

@Carl Thanks for your reply. My sagecom is a dual purpose gateway, as its described as an ADSL ROUTER at their manual.

 I've checked their manual, I am not sure if I can set it as bridge mode. 

Suppose if I set it as bridge mode, would I be able to use all advanced features of Deco M5? ex: Parental Control. Would I still have to connect Deco to it ?

Also, what modem do you advice if I could not set my sagecom to bridge mode? 

@samir_sabri 

Yes if the device can be put in modem only or bridge mode then the Deco can be used in routed mode and you will have access to all of its features.  We haven't sold ADSL products in the US in quite some time.  But for a modem i would recommend any ADSL2+ modem that is just a modem.  If you can find one with a 10/100/1000 port even better.