There is nothing much that can be done in this situation. You will have to contact old tenant global admin and get your account deleted.
Post deleting your account, you can again provision it in new tenant and then you can instance in developer tenant.
If you do not know the global admin of your old tenant you can follow below steps to know global admins.
You can use PowerShell as well you will have to authenticate yourself first. For authentication again it will prompt for MFA to be completed.
I would suggest you ask one of your colleague to perform below steps and share the global admin details with you so that you can contact admin and get your MFA registerd.
• Open Windows PowerShell as administrator.
• Run command “Install-Module azuread”
• Once installed you can run command “Connect-AzureAD” and enter user credentials once it asks for.
• Once you login, you can run command “Get-AzureADDirectoryRole”.
• From the output you can copy the object ID of Global administrator
• Run command “Get-AzureADDirectoryRoleMember -ObjectId "Paste the object ID of global admin that was copied earlier"
• You will get the list of users with global admin role assigned.
Now you can contact any global admin from the list and ask him/her to delete your account from the tenant.
Let me know if you have any further queries
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