It is clear that you haven’t read Srimad Ramayana (original verses by Valmiki). Otherwise such doubts wouldn’t have arose in your mind. Also, it is a popular misconception (fed repeatedly by people with vested interests) that Ramayana considers South Indians as Asuras. Quite the contrary - please see the following verses about South India referred in Ramayana.
नदीम् गोदावरीम् चैव सर्वम् एव अनुपश्यत |
तथैव आन्ध्रान् च पुण्ड्रान् च चोलान् पाण्ड्यान् केरलान् ||
Nadeem Godavareem chaiva sarvam eva anupashyata |
Thathaiva Aandhraan cha Pundraan cha Cholaan Pandyaan Keralaan ||
Kishkinda Kanda Chap 41
Please see the references to the South Indian kings.
तस्य आसीनम् नगस्य अग्रे मलयस्य महोजसम् ||
द्रक्ष्यथ आदित्य संकाशम् अगस्त्यम् ऋषि सत्तमम् |
Thasya aaseenam nagasya agre Malayasya mahojasam ||
Dhrakshyata aathithya samkaasham Agastyam rishi sattamam |
Kishkinda Kanda Chap 41
This is the mountain range in Tamilnadu (eastern slopes of Western Ghat). The Kutralam falls is in these ranges. The Kutralanathar temple was worshiped by Agastya rishi
ताम्रपर्णीम् ग्राह जुष्टाम् तरिष्यथ महानदीम्
Kishkinda Kanda Chap 41
Tamaraparani river is still flowing in Tamilnadu.
ततो हेममयम् दिव्यम् मुक्ता मणि विभूषितम् ||
युक्तम् कवाटम् पाण्ड्यानाम् गता द्रक्ष्यथ वानराः |
Tatho hemamayam divyam mukthaa mani vibhooshitam
Yuktam Kavaatam paandyaanaam gata dhrakshyata vaanaraa: ||
Kishkinda Kanda Chap 41
This is the most clinching evidence.
Here Sugriva, while instructing his vanaras who are proceeding towards south, mentions that they will come across a city called Kavata, which is protected by Pandyas. He says that it is covered with Gold, adorned with precious gems (and he never mentions the word Asura ever when he is talking about these places).
As per the Sangam literature in Thamizh, there were 3 major Thamizh Sangams (a conference of poets, patronized by Pandiya kings). The first one was in a city called Madurai (not the present day Madurai, but another city with same name which was further down south, destroyed by Tsunami). When the old Madurai was destroyed by Tsunami (or maybe raising sea level due to global warming), Pandiyas moved northwards and created a capital called Kapatapuram. It is clear that Valmiki was referring to this capital of Pandiyas as Kavata.
I hope you are now clear that:
South Indian kings were mentioned in Ramayana.
They were never mentioned as asuras.
There is a logical connection with what is mentioned in Valmiki Ramayana and local Sangam literature about the lost land in southern part of India.
Source