That sounds similar to this issue where a user’s network was blocking access to pypi.org: Failed to install Python 3.5.3 - #48 by sergio.agostinho
You might try some of the tests in that thread to see if that is the case. If so, then you are correct and pip3 won’t work for installing the dependencies while on the network. So your solution, or moving the Pi outside the network to install, then adding it back to the network are probably the only solutions for now. If a lot of users are seeing this issue we might try to come up with a workaround in the agent install.