[Sprint LG G2] AzAssassin:
First off, I hope I am putting this in the proper section. The LG G2 I have is an original Sprint-branded device (LG-LS980), but it has been rooted/modded/flashed to be compatible with T-Mobile and I am hoping for some help with getting the phone to receive T-Mobile's LTE signal, so I wasn't sure what LG G2 section to post this in (Sprint's or T-Mobile's). If this is not the right place, please let me know where I should post it and I will be glad to move it to/re-post it in the appropriate section.

With that stated, I bought this phone from someone off of Craigslist two days ago. It was listed as an "Unlocked LG G2" and when I texted the guy and asked him what carrier the phone was for, he told me it was for T-Mobile; so I believed/assumed it was a T-Mobile LG G2 device that was also unlocked. When I met the guy in person, he was very friendly and gave me time to check out the phone myself and to test my T-Mobile SIM card in it. When I turned the phone on after inserting my SIM card to make a test call, the 'T-Mobile LTE' screen was shown during the startup process, which further led me to believe that it was, indeed, an actual T-Mobile LG G2. So, I bought the phone for $260. After I left and got to use the phone for a while, I noticed that there was no T-Mobile logo on the back of the phone, so I Googled the model number (LG-LS980) and realized that it was actually a Sprint device. After realizing this, I also noticed that the phone only displayed the '4G' logo on the front screen, which led me to believe that it was not picking up an LTE signal (I am not familiar with the LG G2, so I wasn't sure what symbol would be displayed). After logging in with my Google account and downloading/installing the SpeedTest.net app, I ran a few tests and it's definitely not getting LTE speeds; it even seems to be getting relatively-slow speeds for 4G/HSPA+ (around 6Mbps Up/2Mbps Down).

So, my question is this: Can this Sprint LG G2 properly receive an LTE signal with a T-Mobile SIM card? And if so, what do I need to do to make this happen?

For the record, the person who rooted/flashed/modded this phone seems to have done a good job in making it compatible with T-Mobile (for the most part). To my knowledge, everything other than picking up T-Mobile's LTE signal seems to be in excellent working order. The camera works nicely; even on 4G, opening webpages is still fast; the music player works; text and picture messaging work properly; and WiFi-calling is even showing up in the Settings as an option (which, to my knowledge, is exclusively a T-Mobile feature. The phone is even showing up as an LG-D801 in the device's Settings.

I also want to add that there is an icon for 'SuperSu Installer' with a message that reads, "Tap to install SuperSU and root your device," but I have been scared to even mess with this feature. I did click the SuperSU icon and a page opened that showed this: "SuperSu is not yet fully installed - your device is not fully rooted. To finish rooting, the SuperSU GUI needs to be installed. This can be done by instaling it through GooglePlay, or by downloading the flashable ZIP and installing it through TWRP." And there is some more to the message below that, but I did not want to take the time to type every word.

So, do I need to go through the process of "fully rooting" this phone in order to be able to properly receive an LTE signal with my T-Mobile SIM card? Or could I possibly damage the phone by trying that process without knowing what I'm actually doing? Or, is completing the rooting process something that needs to be done regardless, even if it's not going to help with with the LTE issue?

Also, I tried to change the APN settings in hopes of getting LTE to work, but the option to change/alter the APN settings seems to be disabled. Does this likely have something to do with the phone not being fully rooted?

I also want to mention that when I went to check for an update in the 'Update Center,' it is showing an 'Android System Update' that is specifically for T-Mobile and it's apparently 149.6 MB in size. Should I try to download and install this T-Mobile update? Or is this something that should not be done without fully rooting the phone with SuperSU Installer?

Also, for the record, it is currently running Android version 4.2.2, Kernel version 3.4.0, Build number JDQ39B, and Software version D80110c

I apologize for writing such a lengthy post, but I wanted to be thorough and try to give as much pertinent information as I could think of. FYI, the LG G2 I have is a 32GB device. I am considering just trying to trade it for a Nexus 5, Galaxy S4 or HTC One, to someone who knows about rooting/flashing/modding, but I figured I would at least ask if there might be a simple fix for the LTE issue before trying to trade/sell it.

I would sincerely appreciate any help that I get! And if I need to make a donation for the help, I would be glad to do so. Also, I can provide pictures and/or screenshots for any detailed information that I may need to provide to help with this issue. Thanks very much in advance to anyone who tries to help me with this issue!!