This fix worked for me for "Failed to join game" error

Press Win+R, paste in "%USERPROFILE%\Documents\My Games\Age of Empires 3\Startup" (or go to "C:\Users\USERNAME\Documents\My Games\Age of Empires 3\Startup" manually)

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Create "user.cfg", put in:

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OverrideAddress="12.12.12.12"

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Replace 12.12.12.12 with your IP from https://www.whatismyip.com, the IP after "Your Public IPv4 is:". Save file and make sure the filetype is .cfg and not .txt

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Done.

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This is assuming you already opened the right ports from your router/firewall, you can check that they are open with https://portforward.com/software/download-instructions/network-utilities/

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(I'm also using GameRanger aka LAN and not official servers so might not work for people trying to play on official servers)