Dell Venue 11 Pro - trouble with PXE boot

Before buying a lot of these tablets (Venue 11 Pro 7130), I have to ensure they can be deployed from MS Configmgr (2012). So far I have no luck getting our test model to PXE boot.

Following the instructions documented by Dell here: 

http://www.dell.com/support/article/br/pt/brdhs1/SLN292679/pt

I have a USB to Ethernet Lenovo dongle based on the AX88772B chipset...which is supported according to the documentation. However, trying to PXE boot, the screen hangs on the message: 

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checking media presence...

start pxe over ipv4

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After a while it then boots on the hard drive. BIOS version is A14.

Any help would be appreciated!