The anti-realist arguments you mention work well in tight knit philosophies. They fare very poorly when you try to combine them with realist style thinking.
For example, when America dropped a nuclear bomb on Hiroshima, no Japanese person had ever smelled, touched, tasted, or heard the nuclear bomb. I'm sure they would have been comforted by the philosophy.
There are three approaches to rectifying this that I can think of:
- Take a complete anti-realist position, and start to come up with theories as to why a belief that the internet exists might be valuable, whether the internet exists or not.
- Use a different trait than "existence" to give meaning to the idea of the internet.
- Use an extended definition of "observe" which includes observing the tool you use to observe the internet. Start with making sure your computer is real (and plugged in), then go from there.