I'm using an Android nexus 5, Android 5.1. Since joining BT I keep intermittently losing data signal. For example I can have full signal with the little H shoeing by the signal bars, but then it randomly changes to and ! Instead of the H, and I can use anying with data. It eventually comes beck. It's not specific to the area as I've been in a whole different county and it still does it. It can't be signal strength either because its full H, and when it eventually comes back it's still full H, it's even still full signal when it shows the !.
I've taken a screen shot but can't see how to post it here.
Are you using the phone indoors or outdoors, ( most likely it' s indoors). Try going to settings - networks - and change to "grp only". Do you get a stable data connection, even if it's only "EDGE" ? Symbol "E" will show.
Try the 2g setting first, and look for an improvement. Then try 3g. 4g isn't officially available here, I.e. It's not shown on the coverage maps - but it's there - sometimes! When it is there it's fast, if you are near enough to the tower.
So, I've been trying your suggestions. Put it on 2G, all seemed fine, even at work where it usually happens. Changed to 3G, again all good, showed the H. As soon as I left the office, which is one street away from the antenna, it dropped to G, then changed to the ! again. Changed back to H about 15 minutes later.
Not sure about the Nexus Phone.... Is your phone connected to wifi, or is wifi enabled at the time this "!" symbol appears? Also on 2g do you have a reasonable data connection, which is usable, even if only at EDGE speeds of 220kbps?
No wiwfi is off, tried 2G and 3G, both do rhevsane thing. Can be fine for hours, then just starts again. You may now, when WiFi connects but doesn't have throughput, it shows the exclamation mark. Well it's the same thing but with mobile data, the exclamation mark appears over the mobile signal indicator.
The exclamation mark is a feature of the Nexus phone Android system. It doesn't show on the Galaxy S5 when the data connection is lost. Looks as if you really are losing the signal intermittently. Not much consolation, but have you looked on the (nexus) forums When the exclamation mark is there go to settings. In the Settings listing there should be an "About" section. In that section look at "status". This should tell you what the network connection is currently.