You didn't mention if you tried a different HDMI cable. I know it doesn't seem like it could cause network issues, but stranger things have happened. If you have access to another network you might want to give that a try - even your phone's hotspot, although keep in mind that streaming video uses a lot of data. Beyond that, I'd suspect a faulty unit.
Are the stick and the Ultra on the same software version? Check under Settings->System->About.
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