I know that detecting asteroids is difficult because many of them we find are the ones that reflect sunlight but can we use something like RADAR to detect others?
The way the question is formulated can lead to contradictory answers.
It can be understood as: since a RADAR can "see" in the dark, can it be used to detect objects that are not illuminated by the Sun (or illuminated but with low albedo)? The answer is YES. Has it been ever used for this purpose? YES. For technical details, cf [Radar detectability of Near-Earth Asteroids] (https://www.naic.edu/~pradar/detect.php)
It can be understood as: Has a NEO been detected by RADAR (and not discovered before)? The answer is also YES. Ex: The moon of asteroid (285263) 1998 QE2
It can be understood as: Can RADAR be used to systematically detect NEOs that escape telescopes' detection? The answer is: YES, in principle, but it would be very very costly. RADARs have to rely on very directive transmitters. It can illuminate only a small portion of the sky at each time.
Yes, radar is one of the useful tools for detecting and observing asteroids.
It is however most effective at closer ranges, like near-earth asteroid, since sending out a radio wave and bouncing it back scales with the inverse fourth power of distance.