i was this problem too, not exaactly the same, but somehow similar. I have 3g connection, the signal bar shows H and somtimes 3G, but the speed is freaking slow, worse than 2g. Initially I thought its something wrong with my phone, been changing ROMS to see if it is the issue, but no. Then I flash the latest baseband / radio, and still doesnt help.
Be reminded I am on 4G plan, but never had any 4g network ( it is not because i'm not in an area with no 4g network )
I was very frustrated and emo at the same time, thinking of RMA the phone, thought it is the radio chip problem.
but then... I called my telco, asking why I'm experiencing slow connection and bla bla, and I don't have 4g network. They keep saying my network is working perfectly, and running normally, I requested them to reset my network connection. They agreed and asked me to turn off the phone for a few minutes then they reset the network for me.
After that, everything is running perfectly.
This was 2 days ago. And guess what, I don't have any 4g connections now again. Stupid telco, I'm going to call them later when i'm free. grrr.
but please make a call to your telco and ask them.
Hope that helps