I was having this problem for several days. The phone was stuck in 3G with one bar of service and would not change. I tried various solutions and searched online but could not figure it out.
The solution was a hard reset. When you shut down this phone it fast reboots, and what you need to fix the problem is a complete shutdown and reboot. You can uncheck the box in the battery settings for fast reboot, or simply hold down the power button for like 10 seconds and keep holding it down without selecting anything until the phone reboots.
I've had similar issues with my data after the latest software update 2.06! My phone will switch on and off between 3G and 4G. then disconnect for a while. Sometimes if I switch to airplane mode then go back, the data connection will not enable. I have to force a reboot sometimes. I too have the 3g only with 1-2 bars and the phone will not transfer any data. It just sits there until I reboot. My previous HTC Incredible 4G LTE didn't have this issue at all. Sucks....I'm an HTC fan, but this is getting real anoying. I'm considering moving over to ATT or sprint just for the HTC One as it seems to be getting rave reviews with no signal issues.
Just out of curiosity, have you tried doing a factory reset?
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