Hello, I hope you are using the latest yocto version 2.0.r1?
Try the below command and check
#ifup eth1
Still if you are facing the issue, I suggest that
add the configurations in local.conf as mentioned at below link then create the image and test.
http://www.digi.com/resources/documentation/digidocs/90001945-13/default.htm#reference/yocto/r_an_cellular.htm%3FTocPath%3DDigi%2520Embedded%2520Yocto%7CSystem%2520development%7CTechnical%2520notes%7C_____1
Still if you are facing the issue, contact digi technical support.
Thanks.