This solution applies to users accessing Azure Active Directory, not to users accessing SharePoint Online.
Conditional Access in SharePoint Online can be configured to use an IP Address white list to allow access.
https://docs.microsoft.com/en-us/azure/active-directory/conditional-access/location-condition
https://techcommunity.microsoft.com/t5/Microsoft-SharePoint-Blog/Conditional-Access-in-SharePoint-Onlineand-OneDrive-for/ba-p/46678