To understand how to configure a subnet assigned by the ISP?

NAT disable is necessary, and should translate it into router mode.
In General, it is easier to put a switch, to stick to his provider cable and two devices on which you want to get these 2 devices and configure them.

Your task does not quite fit existing equipment
You need And nat (office) and direct routing (for phone)
It will in the best case it would be possible to implement on dir-320 with firmware from ASUS enthusiasts by zapilivanija configuration to the state when the interface is not much to manage the work. Or to put on your existing iron open wrt unless she was willing. Judging by Google — will not allow, but this should clarify further.
The alternative is set up on one machine NAT, when she's the one stuck separate patchcords to your router (in router mode) and this interface will hang a public address room 1, and the phone is also plugged into the router, and will have a public address number 2. All the rest go through the machine with NAT (in her role however could be another dir-100)

In the router mode will work
Turn Dlink in роутер1, list those on the WAN primary address (which it is now) on the inner — list the address given to you gatway of the grid (you have spelled 192.168.1.1), for example 178.49.137.1
In the router stick another router (роутер2) in the NAT, and the phone
On роутере2 enter the WAN address 178.49.137.2
on the phone — 178.49.137.3