In fact you can install it from a USB drive. You have to unzip the .zip file and put both of the files from the .zip onto the USB drive. You will then see an option in the medialounge main menu saying "new firmware update" or something like that.
Many thanks for the confirmation that it is possible to do the update using the USB!
I would like to mention two important details for the update to take place:
The DSM-750-VerInfo.txt file must also be on the USB drive and both files must be on the root of the drive and not in a subfolder!
My first try was only placing the update file and not the txt file. My second try was copying the folder over which also failed. The third try was both files stored on the root.
There is no mention of this method of performing the update in the manual and also in any of the knowledge base entries! I guess DLink developers must think this information is gained by ESP!
After update 1.01.16 was installed the internet connection is now connected. The initial problem is now solved!