Try this. Go to Settings. If you have tabs at the top of the screen (e.g., "Connections," "Device," "More," "All") then you are missing a Samsung update that was released April 2015.
I struggled for more than two months with poor signals, no signals, and lost data connections.
I spent more than 6 hours on the phone with Verizon support personnel.
Nothing worked. Until someone in "Tier 2" of Verizon tech support identified the problem.
Why did it take so long? Did Verizon deliberately withhold this update, to frustrate Galaxy S4 owners and force them to upgrade to a newer phone?