Signal issues anyone?

I used to have an S4 and upgraded to S5 on AT&T. All is well, except that I am seeing strange signal issues. I had written these off as issues outside of the phone. However this past weekend I went camping with family at Detroit Lake, Oregon. My wife with S4 on AT&T had good HSPA signal. I was barely getting GPRS. No amount of rebooting, resetting would get me anything better than GPRS. I had to keep borrowing her phone to tether. I am back home now and I keep comparing my S5 signal to my wife's S4. She always has it at about 10 dbm better than me. I measured at several different locations in the house, with and without the cases on. None of it makes a difference. My S5 always has a weaker signal. I have some serious amount of work related travel coming up where I'll be on the road for a month. I am guessing that I cannot take the phone back to an AT&T store to swap for another one. It is about 45 days old. Is warranty repair the only option?

I am hoping that expecting my S5 to work as well as my S4 is not asking too much.

I just wanted to provide a quick update on this. I went to the local AT&T store and they suggested that I trying changing the SIM. It does appear that I was using a rather old SIM. And maybe it did not connect with all the towers. I am not sure. However the new SIM does appear to be working a little better. I'll have to keep an eye on this to make sure that it is not a fluke.