Welcome to the community!
Sorry to hear you might not have understood how post-paid plans operate. You should note that the service start date occurs when you initially activated the line (ie purchased the plan), not when you first use the services (see Terms & Conditions; section 2b). Once the line is active, you pay for the services whether you use them or not. You could activate a line and not put the SIM in a device for six months; you would still be responsible for paying for the six months of service.
You should also note that with post-paid plans, each bill would contain monthly plan charges for the upcoming month plus any additional charges (ie overages, long-distance etc) from your previous month.
Hope this helps 😀