DIAGNOSIS The WiFi setup process on these is finicky. It's almost always because your phone's WiFi is trying to be smarter than the device during the pairing handshake.
FIX First, factory reset the dehumidifier's WiFi: hold the 'WiFi' button on the control panel for 5 seconds until the light blinks rapidly. Now, on your phone, go to WiFi settings and connect manually to the network named something like 'GE_Dehumidifier_XXXX'. It will have no internet. THEN open the SmartThings app and follow the add device prompts. Keep your phone very close to the unit during this.
IF THAT DIDN'T WORK Your home network might be the issue. The APER50LZ only works on 2.4GHz WiFi. If you have a dual-band network with the same SSID for both 2.4 and 5GHz, it can confuse the device. Temporarily disable the 5GHz band on your router during setup, or create a separate 2.4GHz-only network for it.
HEADS UP Once connected, if it frequently goes 'offline' in the app, it's usually a weak signal. These things don't have great antennas. Putting it in a basement corner far from your router is a recipe for spotty connectivity.