Just book a UP/DN auto for Dhanushkoti from Rameshwaram.
I think the road till Sangam is complete now.
If not then you have to change for Maxi service available at check post Dhanushkoti, They take you up to Sangam area spent some 15/20 min there and return again take your booked auto for Rameshwaram.
En route you can visit Kodand Ram temple.