Basilisk network piggybacks on your windows network, so if networking works in windows, you should be able to get it running in Basilisk
Lets run through your settings:
-You have selected "slirp" as network protocol in the GUI.
-You have used the Network selector in OS 7.5 to select Open Transport for networking
-You have restarted
-You went to the TCP/IP control panel and selected Connect via: "Ethernet" and Configure: "Using DHCP Server"
-You click Edit/User Mode, Choose Advanced. Then click Options in the TCP/IP panel, and deselect "load only when needed". Save all settings.
Restart Basilisk. Please report back what it says in the TCP/IP control panel at IP address etc..
I don't have network selector in my OS 7.5.5. Also, when I upgraded I lost tcp/ip control panel.