This is a knowledgebase item. Hope it helps you out someday.
The issue
Changes have been made to the default settings in the Azure AD cross-tenant policies. You want to revert them, but there is no button in the Azure portal UI to do that (at the moment of writing this article)
Solution
This can be done using the Graph API. The easiest way is using Graph Explorer. So, how does that work?
Browse to https://aka.ms/ge, and make sure that you are signed in. First, check if you have consented to the Policy.ReadWrite.CrossTenantAccess permissions.
Then, use this API request:
POST https://graph.microsoft.com/beta/policies/crossTenantAccessPolicy/default/resetToSystemDefault

If successful, this action returns a 204 No Content response code. {}
The default settings are now restored:
Inbound access settings
Type | Applies to | Status |
B2B collaboration | External users and groups | All allowed |
B2B collaboration | Applications | All allowed |
B2B direct connect | External users and groups | All blocked |
B2B direct connect | Applications | All blocked |
Trust settings | N/A | Disabled |
Outbound access settings
Type | Applies to | Status |
B2B collaboration | External users and groups | All allowed |
B2B collaboration | Applications | All allowed |
B2B direct connect | External users and groups | All blocked |
B2B direct connect | Applications | All blocked |
Stay safe!