So they sent you a new one
Likely did not charge you for it
You suspected it would not work - therefor --what?
Living proof that no good deed goes unpunished.
Try this:
DSMX rx are also DSM2 compatible
Which means--
During a binding sequence - the tx sends out a signal -(in this case a DSM2 signal)
The DXMX rx which was previously set to bind -and powered up- is waiting for a DSMX binding signal
BUT if it only sees a DSM2 binding signal - then it accepts that one
Just that simple