If you're browsing on your Samsung Galaxy S5 in a wireless network with a weak signal, then it may be that this automatically switches to the mobile data connection. This can be very annoying, because usually the signal of the wireless network would still be enough to surf in it. Cause for the automatic conversion of wireless mobile data connection is activated option in the Android settings of the Samsung Galaxy S5.
This is: "Intelligent AC line"
It was intergriert on the Samsung Galaxy S5, so will automatically switch between wireless and mobile networks, such as LTE to build up a stable network connection.
To disable the intelligent network change on the Samsung Galaxy S5 proceed as follows:
Select the first thing the mobile data connection. This is necessary because otherwise the setting is grayed out and can not be canceled. Have your mobile data connection on the Samsung Galaxy S5 enabled, navigate to Menu -> Settings -> Wi-Fi. Right at the beginning of the page you will see the option "Intelligent AC line".
Remove the check mark to obtain a not so stable wireless connection of your Samsung Galaxy S5 with the router still upright. Then your smartphone should no longer automatically switch between wireless and mobile Internet.