How to use this like command prompt
Configure a device compatible with SSH, Mosh or telnet to use one or more of those platforms. This device will become known as your server. Set up your network so you can access the device via your iPhone or iPad, preferably with some form of port forwarding. Now, add that device and its credentials to Termius. When you select that device from the hosts list, it will remotely connect to the computer in question and log in to it. From here, you are presented with a full TTY in which you can run any commands your server contains, and any changes you make will be sent back to your computer.
Termius is not a standalone terminal, but rather an SSH, Mosh and telnet client. Without an appropriate server, it is useless.