I have this weird problem with my S5. Usually, my internet connection is H+. Every once in a while the phone seems stuck with H and the internet stops working.
When that happens I have to stop the mobile data and turn it back on again or turn flight mode on and off. This worked every time.
I noticed that the HTC M8, M7, crApple 5S had a better connection stability where I live than my Samsung phones. However my S3 and S4 never had this problem where I have to manually do that to get them to reconnect.
The S4 also kept disconnecting, but it would reconnect by itself. Until one day when it got stuck on E and only at the Samsung shop were they able to fix it. I hope the S5 isn't following on that path as well.
I have tried to set the network just to WCDMA or just LTE/WCDMA autoconnect or WCDMA/GSM auto or LTE/WCDMA/GSM auto. And none seem to fix it. I think maybe on WCDMA (NOT autoconnect) it's better, but not sure if it's not placebo.
Any ideas what I could do and why this is happening? This happened with the stock rom, happens after rooting as well -no surprise, but worth a mention.