Ebay phone: Need some help understanding this OS and how to upgrade

Hello,

I am new here but I have been reading ALOT. I purchased a Samsung Galaxy S5 SM-G900P off ebay and it came rooted with Kitkat 4.4.2 installed. Although it works fine in Canada in general, it's been causing me a TON of headaches. The biggest problem is roaming...I have the "you have lost data connectivity because you left your home network with data roaming turned off" error, which no one seems to have a solution for online. Sim cards in the US didn't work at all but my Canadian one is fine and no issues in Central America, so it is a good travelling phone in general. There are a ton of Chinese characters that populate the phone.

So I'm wondering what the best step would be for flashing a new ROM? How do I do a ROM backup on a phone that is already rooted? Is it safe to install recovery onto the phone? Is there some way to tell if one is already installed? Basically I would like to flash one of the custom Lollipop or Marshmallow ROMs, but above all I need to fix the roaming issue ASAP.

Phone info provides the following:
Phone Model: SM-G900P
Device Type: klte
Product Name: klte
Country of origin: not listed
Original CSC Code: CHN
Firmwares CSC Code: <root required>
Active CSC Code: CHN / TGY
CSC Country: USA/CN
Bootloader/PDA/CSC/BASEBAND version: G900PVPU1AND2
Root Existance: Yes
Android Version: 4.4.2

Any help on some steps to backing up/flashing and upgrading to Lollipop would be fantastic. The tricky part is understanding what has been done to the phone and what I can do to not screw it up lol.

Cheers!

Would like more understanding on rooting a Samsung Galaxy Tab 3 10.1

If anyone has info or is willing to chat wiith me that would be highly appreciated.

Understanding CSC

Hi Guys,

I wanted to understand a little bit about CSC code. I have a n900w8 canadian and I am on the Wind network. That makes the code GLW.

Now I have installed a tmobile rom on the phone and Phone info app shows that
firmware csc =TMB ( for tmobile)
orginal = GLW
Active CSC= TMB/GLW
CSC country= USA

What does CSC do exactly? Does it make a difference in the signal and data connection? or does the APN configures that?

What would be the most optimum setup for CSC to avoid any kind of signal problems?

Thank you in advance for your help.

Understanding Aspect Ratio (for dummies)

Hello everyone,
I was wondering if someone would be kind enough to show me how to adjust the aspect ratio.
Here are the specifics: I have a Samsung T350 tablet(rooted) and it has built in already the screen mirroring feature. Now, I also have an LG webOS UHDTV (48.5in diagonally). Now, if I send a video file directly to my TV from the tablet (using Quick Connect) aspect ratio adjust automatically. When using either my MX Player Pro or the built in player via screen mirroring (for streaming or simply for better navigation controls) the mirrored result on my TV is small and square looking. I'm certain there is a setting that I can avail myself of. Unfortunately, the Aspect Ratio settings on my TV seem to be disabled temporarily when using Screen Mirroring. I know MX Player has the ability to customize aspect ratio, can i utilize that in some way?? I hope I explained this problem correctly. My apologies if I didnt.