I recently purchased a Red Nexus 5 for straight talk and have been a straight talk user for almost 3 years. The only phone I ever used with them before the Nexus 5 was a flip non android phone that happened to be Verizon tower based. The Signal was always 4-5 bars withe flip phone (Verizon) even though I live a rural area. However when getting the AT&T Sim card connected to the Nexus i found that it was a 1 bar or less on coverage (Horrible signal!), but the signal was only the beginning of the problem.
First off let me say I would not wish the experience of straight talk customer service on anyone! Since my last phone was a Verizon and my new phone at&t the web site wouldn't let me port my number over (according to the rep) and that she would do it over the phone. Well all she did was activate the phone with a new number and then told me i had to wait till next month to get it ported. Yeah this went over well..... So I ended up getting my number ported over to Google voice after spending another 20 bucks and forwarded it to my phone. Only problem with Google voice is i don't receive any MMS from them.
I have manged to get my phone working with 2 bars (Sometimes even 3) but i have a feeling straight talk isn't using all of at&t's towers. The AT&T coverage map however shows great signal at LTE speeds in my area. Next month I will be switching over to AIO and see how they fare.
Heres a list of things I did to improve my Cell Reception with Straight Talk.
1. There is a List of Modem Drivers
on XDA that I downloaded and flashed each one separately after testing the signal strength using an app called Network Signal Info
(Lower dBm is better). After a tedious hour of flashing each driver and monitoring the dBM i found that the .15 Driver (D820 KRT01B) was what improved the reception the best for me with .17 a close second. This of course does require root.
NOTE: PLEASE!!!! Backup your EFS by following the guide located at the above "Modem Drivers" link. You do not want to lose that info as it's married to your phone.
2. I Found a set of AT&T APN settings that were different from the rest I saw posted here. Not sure if it improves for you but they seem to work for me.
MMS Proxy: 188.8.131.52
MMS Port: 80
APN Type: default,supl,mms,hipri,agps,fota,dun,entitle
APN Protocol: IPV4
APN Roaming Protocol: IPV4
<Insert Standard Disclamer Here> aka "Use at your own Risk, Im not Responsible)