You can quickly reset a Samsung A3 smartphone by booting into recovery mode (e.g. when you're locked out):

Turn off the phone by holding down the power button (right side) and then select to power off.

Hold down Volume up, Home and Power buttons to power the device on. Keep Volume up and Home down until you see the Samsung logo. After a while you will get the recovery menu.

Use the volume buttons to navigate in the menu (up for up, down for down). Select wipe data/factory reset, then press the Power button to confirm. Use the same keys to select yes if an additional prompt appears.

The reset takes a very short time after which the recovery menu will be displayed again. Either select rebo  ...

If your Samsung Galaxy S5 may not find or be able to connect to a specific 5Ghz wireless network, you may want to check which channel the network is operating on. The S5 has problems when trying to connect to DFS-channels (Dynamic Frequency Selection).

DFS-channels are basically every channel from channel 52 and up. Try setting your AP to a channel between 36-48 (inc.), and then try reconnecting your S5.