Sidhagar is believed to be the distorted version of Siddh gana (divinely qualified person). As per Valmiki Ramayana, groups of rishis lived on the bank of river Saryu. Perhaps this is the same place. Ancient Ramghat is located here. As per the folklore muni, Vishwamitra had gone through this route.
Valmiki Ramayana 1/22/11 to 24 Manas 1/205 doha to 1/208/2