signal strength of SK2 always reports zero

Hi,

I has a AT&T iot Starter Kit(2nd Gen). I installed python2.7 (version2.7.11) to board. I wanted to read the signal strength of SK2. But it always reported 0. Follow are my python code and report log data. 

import iot_mal

mal_sys = iot_mal.system()
mal_wwan = iot_mal.wwan()
mal_network = iot_mal.network()

wwan_radio_mode = mal_wwan.get_network_radio_mode()
wwan_band_info = mal_wwan.get_band_info()
wwan_serving_system_status = mal_wwan.get_serving_system_status()
wwan_radio_info = mal_wwan.get_radio_info()
wwan_lte_ca_info = mal_wwan.get_lte_ca_info()
network_connection_status = mal_network.get_connection_status()

Report log data

wwan_network_mode: {u'errno': 0, u'errmsg': u'', u'radio_mode': 4}
wwan_band_info: {u'errno': 0, u'band_info': [{u'radio_if': u'LTE', u'channel': 5110, u'band': u'E-UTRA Operating Band 12'}], u'errmsg': u''}
wwan_serving_system_status: {u'cell_id': 18576911, u'cs_state': 1, u'level': 0, u'errno': 0, u'radio_mode': 4, u'mcc': 310, u'provider':
                             u'048 26', u'lte_rsrp': 0, u'pci': 327, u'state': 1, u'signal_strength': 0, u'ps_state': 1, u'lac': 0, 
                             u'tac': 4613, u'roaming': 0, u'mnc': 410, u'errmsg': u''}
wwan_radio_info: {u'radio_mode': 4, u'rx1_pwr': -75, u'errno': 0, u'lte_rsrq': 0, u'lte_rsrp': 0, u'gwl_csq': 0, u'lte_rssnr': 0, 
                  u'rx0_pwr': -82, u'lte_signal_strength': 0, u'errmsg': u''}
wwan_lte_ca_info: {u'errno': 0, u'state': 0, u'errmsg': u''}
network_connection_status: {u'connection_time': u'00:00:27:24', u'signal_strength': 0, u'radio_mode': 4, u'errno': 0, u'provider':
                            u'048 26', u'lte_rsrp': 0, u'state': 3, u'data_bearer_tech': u'LTE', u'ip': u'10.192.195.228', 
                            u'roaming': 0, u'level': 0, u'type': 0, u'errmsg': u''}

Thank you.