Asanga was Yadava He was son of Playoga hence he was also called as "Playyogi" .Asnaga was rich king and also a Rajarshi . Shashwati was his wife and daughter of Sage Angirasa.
H.H Wilson in his Rig-Veda Samhita
is providing following explanation.
The Rishi of the thirtieth and three following stanzas is Asanga ,the
son of Playoga ,who having been change to women by the imprecation of
the gods ,recovered his manhood by repentance and the favour of
MEDHATITHI to whom he gave on that account abundant wealth and whom he
addresses in praise of his donation. In the thirty-fourth stanza
SHASHWATI congratulates her husband and is therefore the Rishi.
And this is Acharya Shree Ram Sharma
is saying (in hindi) in his translation , which is same as above.
Shaunaka in his work called Brihad-Devata translated by Arthur Anthony Macdonell give us a brief description based on bhashya of Sayana.
The story of Asanga is relate by Sayana on RV. Viii .1.1 and 34.
Asanga the son of king Playoga ,was , he relates ,owing to a curse of
the gods turned into a women , but afterwards was by the favouur of
the Medhyatithi , restored o manhood by te power of penance. He
consequently bestowed much wealth on the seer (30-33) and was praised
by his wife Sasvati ,daughter of Angiras .
Rig-Veda just gives us some glimpse of the story , but as it is not described in detail by the commentators so for the first part , I am here taking help of one book named Samlee's Daughter: A Novel - By Vivek Iyer , where this story is explained bit further and why the Asanga was cursed by the gods.
The story is this. Asanga , son of Rishi Playogi engaged in Vedic
sacrifice not out of ritualistic feavor but from dispassaionate and
therefore unbounded liberality. In consequence , on one occasion , he
made the mistake of using improperly cut or improperly prepared , wood
for sacrifice .Fotr this act ,more becoming a sluttish maidservant
than a knower of vedas , he was condemned to lose his Bahminhood and
turn into women.Later Rishi Playogi Beseeched the Lord to end the
prohibition on the use of windfall wood , or such or such as partially
consumed by insects , so that , in future , no innocent should suffer
The Brief story- :
Once from the mistake occurred by King Asanga in performing yajna
i.e. offering wrongly cutted wood or fallen by wind ,(eaten by insects) in sacrifice gods cursed him to loose
his manhood and he turned into women or impotant. Shashwati became
depressed by her husbands condition and practice penance. Sage
Medhyatithi restored his manhood and ASANGA and SHASHWATI praised the
sage . SHASHWATI also congratulates ASANGA in the last Mantra. Asanga
gave ten thousand horses and cows to Rishi Medhatithi for this as
described in Mantra 33 .
NOTE -As your question is enquiring about the story of Indra restoring the vitality of Asanga and the sukta is about Indra. But the story part is just of 4 Mantras (30-34) and in those its said that , Rishi Medhatithi restored the Vitality and the couple thanks him by gifting the horses and cows , So the mentioning of Indra restoring it i not present in sukta. But the sukta sounds like gods restored it , the credit goes to Rishi.