ADSL2+ would not be a problem in itself. But the Asus RT-AC68U has no modem "on board".
This router must be connected to the WAN port of the cable modem of the cable network provider (CablecomUPC, EBL, etc) or to a DSL model (Swiscom, Sunrise, Salt, etc). The modem should be in bridge mode and not perform any router activities itself.