Hello Kristin,
If you go to Settings > Accounts > Your info, is the custom domain address listed here?
If so, Go to Settings > Accounts > Your info
Click on Sign in with local user instead. follow the steps and finish the rest.
(If you don't see the "sign in with a local account instead option, click on "Stop signing into Microsoft apps automatically" first, then relaunch this page. )
Restart the computer (Do Not skip)
Go to Settings > Accounts > Your info
Click on Sign in with Microsoft account instead. follow the steps and finish the rest. Sign in with the outlook.com address instead.
Restart the computer (Do Not skip)
See if it works.