Long-Time Customer Paying $100.90 While 1 Gig Is Advertised for $50

Hello Xfinity Team,
I’ve been a customer since 2023, and my bill just increased to $100.90.
I found a 1 Gig plan advertised at my address for $50/month with a 5-year price guarantee. However, customer support only offered me 300 Mbps for $70.90.
I’d love to stay with Xfinity. Could you please help me get the advertised $50 Gigabit plan or an equivalent offer?
Thank you!