No rule for this one
No ending or word group predicts the article of Handy. Learn it as a pair and say it out loud: das Handy.
das Handy
It's das Handy. Handy is neuter, plural die Handys.
Mein Handy ist neu.
My mobile phone is new.
No ending or word group predicts the article of Handy. Learn it as a pair and say it out loud: das Handy.