A quick look at the basic specs shows that that ASUS model does not have a modem inside, the give away will be that it lacks an RJ11 socket on the back to connect it directly to a phone line, only power in, ethernet and USB.
A lot of the devices that are referred to as routers are all in one devices modem/router/firewall/wifi access point and so on .
As the device lacks a modem you will need a modem to connect between the phone line and the router.
you could look for a cheap or a used modem - most will be all in one units, however you should eb able to put some of them in modem only mode, but as it is you will need another box to act as a modem
