That depends on the date your internet subscription becomes active. Perhaps a table would make it clearer:

 

Internet subscription activated on...

Billing date

Between the 28th of the current month and the 3rd of the following month

The 4th of each following month

Between the 4th and 11th

The 12th of each following month

Between the 12th and 19th

The 20th of each following month

Between the 20th and 27th

The 28th of each following month