Most modem-router combos support "bridge mode" which allows you to connect your own router's "Internet/WAN" port to one of the the modem-router's ethernet ports and have it initiate PPPoE to your ISP.
If that doesn't work, you can also just connect the modem-router's ethernet port to a LAN port on your own router, and just disable DHCP/DNS on the router.
My experience is that the DSL modems only contain mid-range routing and wi-fi equipment (10/100 switches instead of Gigabit, 802.11 b/g/n that run with one antenna instead of six and only on 2.4 GHz band). It's adequate to get things "working" but if you pay for a faster VDSL plan (usually 25 or 50 Mbps packages) you can see improved Wi-Fi performance with the right third-party router and laptop/phone.
If that doesn't work, you can also just connect the modem-router's ethernet port to a LAN port on your own router, and just disable DHCP/DNS on the router.
My experience is that the DSL modems only contain mid-range routing and wi-fi equipment (10/100 switches instead of Gigabit, 802.11 b/g/n that run with one antenna instead of six and only on 2.4 GHz band). It's adequate to get things "working" but if you pay for a faster VDSL plan (usually 25 or 50 Mbps packages) you can see improved Wi-Fi performance with the right third-party router and laptop/phone.