Today, the concept of "playing online" has evolved. You don't need a physical link cable or a Wi-Fi adapter that hasn't been manufactured in fifteen years. Through the magic of emulation and browser-based technology, you can now directly in your web browser.
If you want to own the original cartridge for your collection, it is available from various retro retailers: Need for Speed: Underground GBA
Here is the secret sauce that most tutorials miss. In the GBA version of NFSU , the multiplayer mode is accessed from the main menu under "Link Play."
A: This is latency. Go to RetroArch Settings > Latency and enable "Hard GPU Sync." Set frames to 0. This trades battery life for stability.