When multiple Wireless Access Points (WAPs) are using similar frequencies with high power settings, it can cause channel overlap, leading to interference and connectivity issues. This is likely the reason why mobile users are unable to access the internet in the lobby. Evaluating and adjusting the channel settings on the WAPs to avoid overlap is crucial to resolving the connectivity problems.
References = CompTIA Security+ SY0-701 study materials, particularly the domain on Wireless and Mobile Security, which covers WLAN deployment considerations.