Trading in and joining the Apple iPhone Upgrade Program?

I have a 13 Pro Max. Somehow in ordering it last year I apparently did something to kick myself out of the Apple Upgrade program, so this phone is not on the plan. I want to be back on the upgrade plan, and also trade in the 13 pro max on a 14. I can't seem to figure out how to do both of those things together. I can find a way to trade it in, but no option to join the upgrade program while doing so. I can also find a way to join the upgrade program, but no option to trade in the 13 while doing so. How can I do both?

[Re-Titled by Moderator]