Hello @Trochosz ,
Welcome to our community.
Since your internet is working for other applications and only Steam is affected, this anomaly may be related to network configuration, firewall settings, or compatibility with the adapter. Here are some steps you can try to troubleshoot:
1. Restart Steam and Your Computer: Fully close Steam (use Task Manager to ensure it is not running in the background) and restart your computer.
2. Check Firewall and Antivirus Settings: Sometimes, new network adapters can cause Windows Firewall or antivirus software to block certain applications. Make sure Steam is allowed through your firewall and antivirus.
3. Disable and Re-enable the Adapter: Go to your network settings, disable the Archer T3U Plus adapter, and then enable it again.
4. Update Adapter Driver: Make sure you have the latest driver for your adapter. Download and install it from the official TP-Link website.
5. Reinstall Steam: If none of the above steps help, try uninstalling and reinstalling the Steam client.