Is it possible that the packet has to be finished with a line feed/carriage return or both?
The sequence of Serial1.write()s is doing what you describe as the packet structure. How do you send it in Hterm? Do you send the hex code string as written in the post? Do you send single bytes using a hex code entry method?
If the former is true, your code might be like this:
Serial1.println(F("73:6E:70:83:00:01:D4"));