Where did you get the device ?
As you say it didn’t come with proper instructions, I wonder if the mifi device is locked to a certain network, not Smarty or it’s parent network, Three, which would result in the “invalid SIM” message.
Another thought - Check your Smarty dashboard, to ensure your Smarty SIM has been successfully activated and you have bought a plan, which allows you to use the Smarty service.
