Problem with the mobile data settings on Samsung Galaxy S5?

[KAZZM4N]

I am just looking for a fix - please don't tell me to inform my carrier. (I don't want THAT hassle.)
Also, I am based in Australia - Info below.

The settings for my mobile data are broken. I go to 'Data Usage', turn on Mobile Data, and the 4G internet on the device works perfectly. However, when I tick the checkbox, the moment I leave the settings app, or that section in settings, I go back and the box becomes un-ticked again, DESPITE the 4G symbol in the status-bar remaining on, and the internet still working as usual. I also Tether my connection with my laptop frequently, and this is a problem as whenever I attempt to turn on USB tethering, an error appears, stating 'WARNING: Turn on Mobile Data'. When I select the 'OK' option, it takes me to the 'Mobile Networks' section, where it also gives me the option to tick the same box as before. This also doesn't work, and become un-ticked as well. Anyone know what may cause this, and what I can do to fix it?

OTHER INFO:
Model Number: SM-G900T
Android Version: 4.4.2 (Kit-Kat)
Carrier: Unlocked (Telstra Mobile)

 

[Rukbat]

I assume you're referring to the Mobile Data tick box. I'd suspect either the APN settings (you can find them online without going to the carrier by searching for telstra apn australia), or a bad SIM card (for which you would have to deal with the carrier - in person).

And here I thought Oz was such a great place. I have no problem going into either of the two stores my carrier has in this town. If the sales associates can't help me, the people in the repair shop at the store that has it usually can, and they act as if they really want to. (But I'd still like to see the scenery down under before I die.)

 

[KAZZM4N]

I actually did the APN settings myself, because of the lack of 4G LTE support. So I'll go and get them checked out in store. Thanks anyway. By the way, Australia is a good place, and Telstra aren't bad, it just takes awhile to go in and get it sorted, and I just don't have that much time to do so.