MC659B intermittent connection

I'm currently working with a customer who is having an intermittent connection issue at one of his locations. They are using I-PAD at other location which works fine. The site in question is only  location that they have MC659B. This Device connects to both WAN (Verizon wireless) and their WLAN (Wireless network). When they are outside of their building, they are connecting to WAN (VZW)... when they in the building, they are using WiFi. When they are connected to their wireless, the cannot access the internet. They can only access their internal Intranet. Customer have tested with I-PAD at this location and it works fine. Question is- What is the best practice or best setting for the device (MC659B) to make the best adjustment between WAN and WLAN seamlessly. Would it be best practice to use Microsoft Connection Manager API to control connection from application or what? or Can I make any adjustment under Network management? Any thoughts?