Please suggest ADSL router combo vs separate

Hi guys,
I am currently using an Airtel 16 mbps connection and am looking to upgrade my current 450TC1 to something substantial. My requirements are:

1. Proper range for a 1000 sqft apartment
2. Wifi AC standard
3. USB support
5. Budget <4000

I am interested in flashing openwrt or similar firmware, but a single ADSL Wifi router has its own benefits. So please suggest routers under Rs 4000. I am confused between: TP-Link Archer C20/Asus AC51U with D-Link DSL-2520U modem OR TP-Link Archer D20 Combo

TP-LINK Archer C20 AC750 Wireless Dual Band Router: Amazon.in: Computers &amp; Accessorie
Asus RT AC51U Dual-Band AC750 Wireless Router (Black) - Buy Asus RT AC51U Dual-Band AC750 Wireless Router (Black) Online at Low Price in India - Amazon.i
D-Link DSL-2520U ADSL2+ Wired Ethernet/USB Combo Router (Black) - Buy D-Link DSL-2520U ADSL2+ Wired Ethernet/USB Combo Router (Black) Online at Low Price in India - Amazon.i

TP-LINK Archer D20 AC750 Wireless Dual Band ADSL2+ Modem Router - Buy TP-LINK Archer D20 AC750 Wireless Dual Band ADSL2+ Modem Router Online at Low Price in India - Amazon.i