Hi! Here follows a post that I made at Reddit describing how to solve this if you already tried everything and none worked out to solve this problem.
It all begun when I, waiting for the Marshmallow update, decided to replace my battery so I could enjoy the full benefits of the new feature "Dose".
After the succeded battery replacement I noted something weird. My 3g and LTE signal were extremely bad, and they were oscillating too. Sometimes I got full bars, sometimes two and sometimes just the empty triangle. Even when I got full bars I couldn't navigate or send something in Whatsapp.
I totally freaked out with this problem since in my country, Brazil, smartphones are a extremely expensive stuff, and I was saving my money to buy the Nexus 5X which was not released here yet.
I tried each and every fix I could find across the web, reverted to KitKat, pressed the backcover, flashed numerous radio images, checked the coaxial cables, tried new simcard and none of those worked out.
After all this I was convinced that It only could be a Hardware issue. I noticed that our Nexus 5 has a auxiliary PCB and this PCB houses one of the coaxial connections of each antenna cables. I was convinced this PCB could really be the reason of the signal problem.
I called a friend who has another Nexus 5 which he even use anymore due a cracked screen and decided to do some transplant.
After the succeed transplant, voilá! I now got full bars of reception in 3g and LTE.
I really think that those coaxial connections at this PCB were faulty even they didn't show any signs of it
I strongly recomend to anyone who is almost giving their hopes up to try this! This PCB is very a cheap part.
Sorry for my english, as you had read, its not my native haha!
Please let me know if this worked out for you too!
Here follows a image of this PCB part: https://d3nevzfk7ii3be.cloudfront.net/ig...