There was significant Roman trade In the Indian Ocean. So Roman merchants in SE Asia is not inconceivable.
I didn't know that the Romans went there.
St. Thomas the Apostle evangelized India. That's why southern India is Catholic with many males named THOMAS.
Google
According to traditional accounts of the Saint Thomas Christians of India, the Apostle Thomas landed in Muziris (Cranganore) on the Kerala coast in AD 52 and was martyred in Myalpur, near Madras in AD 72. ... It was his mission to espouse India to the One-Begotten.