The range test function requires one of two things to function. First it is it needs a way to short data in to data out or use the loop back cluster ID. Next it needs to use API mode (in the XCTUNG software only).
Since the Xtend does not support Cluster ID's and End points as that is a Zigbe function, that option is out. Next, a USB interface does not allow one to short data in to data out. Only an RS232 or TTL level option does. So in this case, the range test would not work very well.
However, you can use the XCTU 126.96.36.199 version's range test from both computers at the same time which would give you the effect of a loop back adapter and allow the range test to report good responses.