Forum Discussion
Script to add global SP admin to all site collections
In this thread https://techcommunity.microsoft.com/t5/SharePoint/O365-Global-Admin-has-no-access-to-recent-SharePoint-Online-site/m-p/30031#M2542 a script was posted to add a admin user as site owner...
I would suggest to use the pnp commands for it.
https://github.com/SharePoint/PnP-PowerShell/blob/master/Documentation/SetPnPTenantSite.md