Flight Instructor reported an NMAC while joining airport traffic pattern. Instructor made an evasive maneuver and continued flight.

Narrative

I was descending in the class D at FDK airport to join the pattern; ATC made a report that there was traffic that had departed and westbound; I saw traffic on my ADSB-in so I started to fly away from traffic toward the left to leave more space for the departed traffic; traffic started flying toward us but we could not visually see them so my student and I were looking visually; I spotted other aircraft very close to us so I made an evasive climbing maneuver to the left instead of the right because aircraft was slightly to our right and did not alter their course. ATC in FDK does not have radar.