I have a Mikrotik Router RB951 Ui-2HnD, when we connect modem then usb port showing "unknown"

We can't found any usb port in system>ports.

How can we add usb1 port under system>ports.
Please help me

You first check if your device is compatible with MikroTik device (with version of RouterOS you use).
RouterOS do not have inside, and never have, all the "drivers" possible for all device on the market.