Just to be clear:
selected correct port type;
By this, do you mean that in the IDE, under the Port menu, the Yun Shield showed up with a network address, and you selected that menu item?
From the error message, it almost seems like the changes to the board.txt file weren't done properly. You might have better luck contacting Dragino directly about this issue, as they are the ones that are recommending that you change the IDE configuration files. They will have a better idea of what's going on with their changes than anyone here. The board settings that they have you add are rather different than what the actual Arduino Yun uses, you would have to ask Dragino why they are different.
Just keep in mind that while an Uno with a Dragino Yun Shield will be able to do most of what a Yun does, it is not 100% a Yun and there will be some details that are different between them. This is apparently one of them. Most of the people here will be familiar with the actual Arduino Yun, not as many will be familiar with the Dragino Yun Shield.