The other trouble for OP is that Frontier no longer has phone customer service. Live chat is the only option.
Glad you figured out how to book it, OP. Hope it goes smoothly for you. Best advice is to act like you’ve done it before and tell the agents how it’s supposed to work. Nicely and very patiently. You’ve paid for the seat, it should be no trouble to let the trombone sit in it. Fingers crossed.