can't find office 365 ATP add-on under add subscription part in billing

Maybe you are looking for the wrong name?  It should be listed under Add-On Subscriptions as "Office 365 Advanced Threat Protection".  I am seeing it in multiple tenants of varying types, although at this URL it says to contact your reseller if you want to add it.  https://products.office.com/en-us/exchange/online-email-threat-protection