Using Classic Outlook on Windows for personal email, calendar, and contact management
Have you tried this?
This feature is available in Outlook for Windows for Current Channel (Preview) users on Version 2004 (Build 12730.20150) or later. Following the registry entry will disable it.
Open command prompt with user rights not admin.
Copy below and press enter. Please confirm if the registry entry esxist after running.
reg add "HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Outlook\Options\General" /v "EnableImprovedCaptivePortalExperience" /t REG_DWORD /d 0 /f