Help with Galaxy S4 -Verizon to T-Mobile

Hello, I have been reading through these forums and am starting to feel a bit of hope. I have a unlocked Samsung Galaxy S4 that I purchased from a friend who is a Verizon customer. I am using T-Mobile (Family Mobile...same thing?). The phone works with the T-Mobile SIM card however, the data is sketchy and the Text Messaging works some of the time, but does not work most of the time. I live in a Metropolitan area with excellent coverage. In fact, my wife and children have no problems with T-Mobile service; however, their phones were originally configured for T-Mobile. I can't say for certain, but it seems like the radio's are either incompatible or that they are configured incorrectly for T-Mobile.

I have never modified a phone's OS or ROM before, but I am familiar with the concepts and I am not afraid. Is there a custom ROM that can be flashed to get all traces of Verizon off of this phone? And can I configure it so that it is optimized for the T-Mobile networks?

Thanks in advance for your help!

Here is some info about the phone:

Model: SCH-I545
Android Version: 5.0.1
Baseband: I545VRUG0C1
Kernel: 3.4.0
Build No: LRX22C.I545VRUG0C1
Hardware Version: I545.06
KNOX: 2.3
Configuration Version: J12.SAM.SCHI545.0