What to do with yellow ethernet cable

Plug it into your computer ... If you just want to use the WIFI you can keep it and not use it for now.

But ethernet connection is better than wifi by a long shot.

Have a nice day :)

As @Dgjf in terms of stability, reliability (and often speed, but less and less with Wifi 6 (AX) and MU-MIMO) nothing beats a wired connection. That cable can be used in multiple ways:

• Connect your own router and put the Fizz modem in bridge mode (although most router come with a cable already)
• Connect your Fizz modem to a switch if you need more than the 4 basic wired ports
• Diagnose speed test issues using a laptop directly connected to the modem (unless the modem is already next to a PC because that cable is darn short)
• Plug any other device that you keep close to the modem such as an ATA (VOIP bridge) or NAS (network storage)
• Strip it and discover how network cables are built

He came with the Fizz modem. In the box you get the power cable, a black coaxial cable and a yellow ethernet cable.