You may be able to use information here along with the correct
Ethernet cable, (not phone modem cable) to internet router, so
the fastest possible connection from the IP's device to your old
Mac could be attained.
•Creating a small Ethernet network
•Solutions for connecting to the Internet, setting up a small network, and troubleshooting
•Using network locations in OS X v10.5 or earlier ...about wi-fi mostly, yet shows:
Mac OS X: How to Connect to the Internet via Cable, DSL, or Local Area Network (LAN)
Mac OS X: How to Connect to the Internet With PPPoE
Mac OS X: How to Connect to the Internet via PPP (Dial-Up Access)
If you are using a phone modem, and dialup, that is not via an Ethernet port.
You can see in the article(s) where Ethernet cable plug is different than phone.
You can use an old AirPort base station as a wired router, I have. That's a
different topic and I've forgotten how I did it; without Support help links.
Not sure if this helps, but I had these bookmarked for several years and had not
tested them recently until just now as I sought to include them here.
PS: if using Mac OS 9 (classic) as a booted system not under OS X, there are
other topics in Support via special search that bring up old articles such as this:
•Mac OS 9: How to Connect to the Internet via Cable, DSL, or Local Area Network (LAN)
•Mac OS 9: Troubleshooting a Cable Modem, DSL, or LAN Internet Connection
Good luck & happy computing! 🙂
edited