You didn't do what I said to do in Reply #25 regarding write(var), print(var), println(var), print(var, HEX), and println(var, HEX). No wonder it isn't working if you are using print(data_start, HEX).
I'm reverting to your code in Reply #24, applying my suggested changes in Replies #25 & #26. I slightly modified your transmitting sketch because I don't have your temperature sensor. I commented out any and all lines dealing with I2C and set data_high and data_low to two values you showed in an earlier screenshot.
I also added some other diagnostics. I can see proper transmissions and a bug that I inadvertently told you to do in the "2 characters to 1 hex value" function. This is using the if(XBeeRX.available) and the switch case structure.
I need to get to bed now, so I'll fully document and explain what I did in the morning. (Along with a potential problem and provide a work-around.) Sorry for putting you in suspense...