Choosing to radiate the RF signal from a GSS5700 WiFi simulator instead of using a direct cable connection to transmit the signal can be because:
When using the GSS5700 with an antenna to radiate the WiFi signal the distance between the DUT and the radiating antenna is very important. With the device just 1m from the antenna the freespace path loss will be ~40dB. Many WiFi devices often struggle to maintain connection with WiFi signals below ~ -70dBm. So although power level settings on the GSS5700 unit may appear to offer a suitable signal level you must also consider the freespace loss to determine whether the received signal level at the DUT is simply too low for appropriate APID acquisition.
Try positioning the DUT close(r) to the radiating antenna and start with a slightly higher power level setting on the GSS5700 than you would perhaps normally consider.
Please also be aware that most WiFi antennas use reverse polarity SMA connector antennas. If you connect this type of antenna to a normal SMA connector you can end up with two connected socket terminals which introduces more loss.
If you are still not confident that your GSS5700 simulator is generating a signal with the expected power level there are some simply checks you can perform. Please see article SOL11157.