The way a speedometer typically works is the speedo cable whirls a bar with a magnet on it, and the magnetic force is sensed by a bar connected th the speedometer needle.
The magnet and the sensing bar are not supposed to touch.
Given that your needle stuck at 20 mph, that might lead me to suspect something in the speedometer body.
You might be able to spin the cable at the angle adapter with a drill, and have someone watch the speedo for you…. But this won’t tell you if it’s in the cable or the instrument body.
You might need to pull the speedo out of the dash to isolate the problem…
Rocky