first off, im proud of you
well, the only thing i can think of is the nexus 5 has a shorter antenna then many other of the high end phones, one of the many reasons why it costs so much less than others. it would be interesting to compare another nexus 5. or have you?
Interestingly, once I got back into an area where I was getting LTE coverage, then came back toward home, I was able to get on 3G on the towers with issues, so perhaps updating the radio fixed it after all, I just had to get somewhere that the phone could figure things out first. I was on the oldest (KFS78N) last night, but I'm going to try KRT16M tonight and see how it goes.