Dear William Wintle,
Thank you for reaching out to Microsoft Q&A Forum.
I am totally understanding your issue that you are having trouble logging into Outlook.com using Mobile Wi-Fi. You tried adding an alias, but it was treated as a second username and didn’t work.
Here are some tips to help you solve the issue:
- Use Microsoft Authenticator
- Step 1: Download the app
- Step 2: Open the app and tap "+" to add an account
- Step 3: Choose "Work or school account" or "Personal account"
- Step 4: Following instruction to sign in or scan QR
- Step 5: Use the app to get your 6-digit verification
- Use a direct 4G/5G mobile network instead of mobile Wi-Fi through a VPN
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