hi,
since you didn't provide the detailed information, so it is hard to determine which actions should be taken to resolve the problem.
but you can advise to the customer to inspect if there are devices which may disturb the wifi signal.
or you can reffer to http://support.huawei.com/hedex/hdx.do?docid=EDOC1100025386&id=EN-US_TOPIC_0116819901&text=Help%25252520Topic%2525253A%25252520WLAN%25252520Network%25252520Optimization&lang=en