We are in version 6.0 jonin minato was a version 1.0 maybe 2.0 ninja. The don't really release ninja from the older versions. The only way you will get jonin minato now is if they put him in recruit or possibly give him away in fire works festival. You do get frags for jonin minato from home interface when you recruit any version of minato. But unfortunately it wont be enough to recruit jonin minato since at best i think you only get 40 frags for max friendship level with him. They have been releasing some of the version 2.0 ninja like madara 5 kage. So he might appear in monthly sign in eventually.