I believe that the answer to each question can be found within our Top Up card documentation: Link.
(Edit)
My apologies, the link above did not answer your last question. To view how much free data remains for the month, please perform the following:
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Open the EasyAccess 2.0 application:

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Select an HMI:

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The amount of free data remaining is equal to 1024MB - ‘Network traffic’:
