Watch the video:
Timestamps:
0:00 - Explanation
0:10 - Steam has maintenance
0:56 - Move to Battle.net
1:11 - Verify game files
1:45 - Try wired
1:56 - VPNs and Proxies
2:20 - Change DNS
3:05 - Flush DNS
3:20 - Slow internet fixes
3:49 - Disable Texture Streaming (Texture Downloading)
Have MW3/Warzone 3 on Steam and get the “Disconnected from Steam” error? This video explains why it’s happening as well as few possible fixes that should get you back in-game.