There are two connection modes - WiFi (WISP) and 3G/4G.
WiFi / WISP
With this mode set (by sliding the Mode switch), the TP Link device behaves like a WiFi Modem. It accepts an Ethernet connection on one side (to the Key Cabinet) and then connects to your office WiFi network access point (known as an SSID). Just like how you might connect your smart phone or laptop to a WiFi network, the TP Link device connects by selecting the SSID and supplying the connection password.
Like the WISP model above, but except for connecting to your office WiFi network the TP Link device has a USB 3G/4G dongle inserted, which connects to the public cell or mobile network.
Note that in addition to the TP Link device, using this approach you will also need to purchase a USB 3G/4G dongle plus SIM card with a data plan.