Hi again,
The E5776 models are 4G/LTE mobile broadband hotspot devices, using Wi-Fi for the local connection.
These devices do not have an Ethernet port. They do have a USB port.
In order to get Ethernet, one possibility could perhaps be to connect the hotspot to an appropriate dedicated mobile broadband router (with LAN ports) via USB. To begin with, one would have to check whether the USB port of the E5776 version in question can handle data traffic (not only power supply). Also, one would have to find a separate mobile broadband router with support for the hotspot and the mobile network.
Another solution could possibly be the use of an AirPort Express (http://www.apple.com/airport-express/specs/) as an intermediary. The idea would be to somehow try to let the AirPort Express join (connect wirelessly to) the 4G hotspot's Wi-Fi network, and then use the LAN output of the AirPort Express for the connection to the WAN port of the Time Capsule. However, a number of conflicts and configuration issues may appear, so be prepared to experiment
Jan