Have I been scammedon the $830 promotional credit?

Can anyone explain to me how the $830 trade in credit works when you upgrade your plan to the new 5G?  Surprisingly, T Mobile can not and I still don't know what they gave me for my trade in value but it wasn't $830.  The man in the store broke it down this way… pay taxes and fees on any phone, trade in eligible device, sign up for the New 5G rate plan and we deduct the $830 from your new phone choice.  You pay the difference for 24 months.  So for an iPhone 14 pro max with tax and activation paid up front, That leaves a $1099 for the phone minus 830 is 269.  That is roughly $11 month for 24 months for the phone.  So why is the monthly $20???