RainOne false advertising: Paid for 2 months instead of 1

The service provider overcharged me when it came to my billing for the rainOne WiFi package. They had advertised that the first month would be free, but they charged me pro-rata for the month that was supposed to be free. I joined the service provider on the 17th of November 2023 and they had stipulated that the next time they will be billing me will be on the 15th, but they debited for two months. PLEASE ADVISE