This is a quick hint in case you suffer from the same issue I had while installing Fedora 26 (alpha).
The installer didn’t manage to connect to my wireless router, a D-Link one. More specifically, it was not getting an IP address from the router. Some problem with DHCP it seems.
If that’s the case, open a terminal (press the Windows key, then type “Terminal” and hit Enter), and type the following command:
sudo sh -c 'echo "send dhcp-client-identifier = hardware;" > /etc/dhcp/dhclient.conf'
Then reconnect to your wireless network. After the first boot – when the system is installed – repeat the command above, just once.
By the way, this is not a new issue. I experienced this on Fedora 25, but curiously at the time the installer (the live system actually) worked fine. Just the installed system suffered from it. Now, with F26, it happened since the beginning. Here’s the bugzilla entry: https://bugzilla.redhat.com/show_bug.cgi?id=1154200.
Hope that helps, happy Fedora!