If you are using the original Monimoto SIM card that came with the device and the app HOME screen looks like this:
You just have to go to the Monimoto app > Settings, press TOP UP SIM CARD and you will be sent to a page where in just a few clicks you can TOP UP your SIM card.
If you can't find the SIM CARD menu on your app and on the bottom you see [V1], it means that your Monimoto is running on an old cloud version and the firmware needs to be updated. Just go here and update firmware to the latest version. Once the firmware is version 0.10.5.61 or later, it automatically applies to the new cloud [V2] and you can see the TOP UP SIM CARD menu.