Sharepoint site external sharing not working

I have tried searching around for this problem but have not been able to find anything.  Recently (within the last month or so) both our internal and external users have been unable to send links to external users.  We are running SharePoint online with a mix of modern/classic pages.  

  When I Share first it works well, We have most of the external clients to share with SharePoint site. All of sudden in 15-20 days, Most of the client's complaint they don't have access and we have to re-share the document again and again. It leads to the frustration of the client. I have even talked to Microsoft about this and they said there is no issue in setting but still the issue is happening again and again. We gave domain-level " domain-level access" permission. Here are the settings screenshots below :

Any help or suggestions would be appreciated. Need urgent help guys.