Does the 'Isha' prayer become valid if offered after the Fajr prayer, days after remembering it was missed?

What you did, making up the 'Isha prayer after the Fajr prayer, is correct, due to Allah's saying: "And establish prayer for My remembrance," and due to the Prophet's (peace be upon him) saying: "Whoever forgets a prayer, let him pray it when he remembers it. There is no expiation for it except that."