Tapas made a mistake which was Hangyum the idol was supposed to be going by Han in present day. Tapas just changed and covered up things. Making things more complicated. Many has been confused. The fan translations group which are closer to the real Korean story are working to get the chapters up. So some of the confusion that exists in the Tapas version will not exist in the fan translations.
Chapter 21 Hanwool knew only the name of Hajin because when Hangyum was being heartbroken in bed saying his name over and over.
Chapter 27-28 is the first time Hajin and Hanwool Mets each other. Which is present day.

Okay so which brother did he meet in high school? Cause I keep getting confused