It’s easy to find the system key location of 2wire routers, the system key is located on the side of the router – it’s the same as the wifi key. However you’ve probably already tried that when resetting the password, but had issues… Which is why you’re searching for the system key.
Here’s the trick: You were probably using the correct system key, but the password you’re trying to make doesn’t match or isn’t long enough or the hint is similar to the password. Close then open your browser before picking another password to set it to. You can then change the 2wire password later to something more simple from the Settings -> Password page, which doesn’t seem to have the same password requirements 😕