Xfinity support

I am an Xfinity Diamond Rewards member trying to switch two lines from T-Mobile. I am attempting to order 2 iPhone 17 Pro Max with eligible trades on the Unlimited Plus mobile plan to qualify for the advertised $1,100 trade-in promotion. The store interface correctly advertises the Pro Max at $2.77/mo, but the online checkout cart glitches out, locks me out of plan selection, and forces the price to $13.88/mo. I need an agent to manually build this cart on the backend so my Diamond status and the correct $1,100 device credits stack properly