The most common reasons for this are either: Data Roaming needs to be enabled or the APN is blank.
To enable Data Roaming on the Huawei E5577: Press the “Menu” button situated on top of the casing repeatedly until “Data Roaming” appears in the router display window, then press the “Power” button once, which is situated to the right of the display. There is a link to a more in-depth article here.
To program the APN: click on the link below, which will take you to a short and easy-to-follow instructional video.
How do I program the APN on my Huawei device?