How to communicate with usm modem in android studio

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Android

Xamarin

I cant send at commands to the modem that connected to my android phone. How to do it?
I can find my usb modem with this code

What I have tried:

int pr_ID, vn_ID;
                pr_ID = vn_ID = 0;
                if (manager.DeviceList.Count != 0)
                {
                    foreach (UsbDevice device1 in manager.DeviceList.Values)
                    {
                        view.Text = "Manufaturer Name= " + device1.ManufacturerName + '\n';
                        view.Text += "Device ID = " + device1.DeviceId.ToString() + '\n';
                        view.Text += "DeviceName= " + device1.DeviceName + '\n';
                        view.Text += "ProductID= " + device1.ProductId.ToString() + '\n';
                        view.Text += "ProductName= " + device1.ProductName + '\n';
                        view.Text += "VendorID " + device1.VendorId.ToString() + '\n';
                        pr_ID = device1.ProductId;
                        vn_ID = device1.VendorId;
                    }
                }