Question about Galaxus Mobile CH Unlimited plan

But now we are being cheated? I redeemed a Black Friday voucher code (12 months for CHF 100) on 22 March 2023. I used this to pay the basic fee for 12 months in advance. Now I have received a credit note, but it doesn't even last until the end of October. What about the months of November until 21 March 2024? I had already paid for these in advance with the voucher. Now I have to pay extra for this period? Is this legal? It's not just me, but also the Family members and other users here. For me, this is fraud!