The first thing you should make sure that the TV is in TV mode, then do a channel search and if the problem persists you should check the tuner tensions.
From here you can download the diagram:
https://elektrotanya.com/samsung_un19-26-32d4003bd_un22-40d5003bf_ln32d403e2d_ln32d403e2dxza_chassis_u59c_u59e.zip/download.html
Hello! Did you check that the antenna conetor is not broken or loose in the tuner?