The Doro HandleEasy 321rc is an infrared universal remote control designed for controlling a device (TV) by entering a code. The Swisscom Box 21, on the other hand, comes with its own remote control that uses Bluetooth Low Energy for the connection and also has an IR transmitter for TV volume control. There is no official statement or confirmation that the Doro HandleEasy 321rc is compatible with the Swisscom Box 21. The Swisscom Box 21 requires a remote control that supports Bluetooth, which is not the case with the Doro HandleEasy 321rc as it only uses infrared. Compatibility is therefore very unlikely.
