This Manu is otherwise called Prajäpati; and Ikşvāku is Manu's son. And this Ikşvāku you must understand, was the first king of Ayodhyā.
मनुः प्रजापतिः पूर्वमिक्ष्वाकुश्च मनोः सुतः। तमिक्ष्वाकुमयोध्यायां राजानं विद्धि पूर्वकम्॥