Enter mode on the S-A-A-2 (via device menu or holding down the JOG LEFT button while powering on).
Calibration is essential whenever you change your frequency range. NanoVNA V2 : Under the Calibration pane, click Clear Measurements SOL (1 port) for S11 measurements or SOLT (T/R) if measuring S21 (filters/amplifiers). nanovna-qt pc software with the s-a-a-2