I have to toggle Mobile Data On/Off/On every time i want to go on the internet using 4G. Otherwise i get the "Page Not Available" in the Chrome browser.
Hate to say it, but based on some of the other issues you seem to be having you may want to try a factory reset if you haven't already. Seems like you are having some OS related issues with your antennas in general and a factory reset may help.
I think you have a bad phone go exchange it for another one. The same things would happen on my first S3. I would lose mobile data and could not get texts while on wifi. It seemed like it had a bad switch. While on wifi my data and sometimes phone would not be active. I fought with it for about a week. I exchanged it and this one works perfect.
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