There is no direct connection from Daejeon to Sejong. However, you can take the train to Jochiwon Station then take the taxi to Sejong. Alternatively, you can travel from Daejeon to Sejong in around 26 min.
From Seodaejeon Station train to Jochiwon Station then taxi to Sejong. 39 min ₩22,803. From Daejeon taxi to Sejong. 26 min ₩34,166. From Daejeon drive to Sejong. 26 min ₩4,661.