Use Open Transport 1.1.
Earlier versions are full of bugs.
OT 1.1 comes with System 7.5.3, System Update 2.0 from Apple. For a PPP connection, use FreePPP 1.0.5. Look for the Apple Extras folder which is installed on your hard drive after installation of 7.5.3. Restart your Mac. Open the Control Panels folder in the System Folder.