Modern external network migration, how to sign into new tenant with admin account.

Ok, so it turns out the Viva engage implementation of the parent tenant is in a different geographical location to the one I chose when creating the new tenant.

The confusing thing is that if I go to "365 Admin > Settings > Org Settings > Organization Profile > Data location" it tells me all my services are located the UK, yet Microsoft themselves (there is no way for a customer to check) have checked an tell me my Viva data is homed in the US. I can migrate Viva to the UK but all my Viva data and communities are lost and I have to start from scratch. To me this is a massive oversight especially with GDPR etc. and UK public organisation needing to keep data within the EU.