XS Max (256gb) Trade in device "in transit" for over a month?

Billing credits are not applied until three billing cycles after the trade in. Since it hasn't been that long yet, there's no "assistance" to provide. If the credits don't start by the 4th billing cycle, then you should file a BBB complaint. 

Just to confirm, you sent your phone in with the separate trade in kit that was sent to an address in Tennessee?

You can use your tracking number on the Pitney Bowes website.   Pitney Bowes website should give you some idea if the package has been delivered to hyla Mobile in Tennessee. Hyla Mobile is the recycler. Your phone may be received, but that does not mean the package will be opened until they are ready to process your phone. It is not unusual for it to take two or three months before you get your credits.

Catch up credits for the missing months are applied when you start getting Bill credits

Update: It's been past 3 months since I mailed my old iPhone to ATT for device trade in and still missing the credits. I have a Pitney Bowes tracking number (which routed the phone to somewhere in Indiana), again, this was sent using the "DEVICE TRADE-IN RETURN KIT" that came with my 14 Pro Max back in late Oct. 

Can someone ATT employee please provide assistance?

It's time to file a BBB complaint. That will get you to someone who can actually get this resolved.

Indiana! 😟

One of my phones did squat in Nashville for 2 full weeks.  I cannot imagine why it would be stuck in Indiana.

File the online better business Bureau complaint as suggested by @MicCheck - twice. The complaint is forwarded back to AT&T upper management and someone will call you