In general - you will be charged a 3 Eur monthly fee (paid for the whole year in advance, as a 36 Eur annual fee). You will receive two notifications before the end of the free GSM service period (which is 2 months). When GSM service expires - you will be able to renew your account from the Monimoto app.
You will be able to top-up SIM card only if Monimoto firmware is 0.10.5.61 or newer and runs on new cloud V2. Otherwise you will not see SIM card menu on monimoto app settings tab.
To check firmware version you should connect via Bluetooth or simply open the app, go to settings and check whether you see [V1] that means old cloud or new [V2]. What's new on V2 and how to update it - find here.