Buying an Iphone13 to Trade-in for Iphone 17 Pro

I was thinking of trading in my IpXsmax for the $350 promo, until i read some posts where people have been buying an Ip13 with locked network (Straight Talk) on Walmart to trade in for the Ip17pro. This way is the most cost effective and almost like getting the Ip17pro for only $200 they said.

*What's the catch for this one and does this work for anyone else? I just recently came to the US for roughly more than 1 month so i'm not familiar with everything here.
I read that network lock is fine and they accept any condition, as long as IMEI is not blacklisted.

Is there anything else they require such as "currently on their unlimited network plan" or anything? I don't want to risk buying a phone just for it unable to trade-in and waste $200.

My current Xsmax is already on AT&T Unlimited Extra® EL plan since i came here last month

P/s: i only research about this after dropping my already rusty Xsmax. Everything is still fine except it's glitching and flashing and bleeding pruple OLED ink now. I initially thought about get this one fixed then trade it in but now here i am. I really appreciate your support because i do have enough money to buy upfront an 17 pro, but i need it to be cheapest as possible.