System and Band Frequency issues. please help

Question

Please bear with me as I am brand new to this. This is the first time on this site/forum so I apologize in advance if I act at all "blonde".
I have a Samsung S4 SGH-I337. 
The phone is Rooted which I was given this way. 
I have a few questions and any advice would be greatly appreciated.  First off, I am unable to access almost all system apps (phone, messaging, e-mail, Google play store, etc.) 
When I click on the app, sometimes the screen just flashes and does nothing, and other times I get a popup box that says "Unfortunately _ _ _ _ _ app has stopped"
Is this due to something being removed when Rooted or is this due to something I have clicked on ? Is this fixable or should I move on to a new phone ?
Secondly,  the carrier I am currently with is not compatible with this phone. Due to this I can not make or receive phone calls due to the band frequencies (1700-2100 is missing on this model).
Am I able to add this frequency and if do, could someone please walk me through? 
Or again, move on to a new or different phone?
Please help and thank you again for your patience

Lindsay mcdaniel · Answer

Whoever rooted that phone probably removed 1 or some of the systems apps. And they neglected to back up the apps on a different computer first, so they had no way of reinstalling the apps when the problem happened.  And when they couldn't fix it they gave away the phone.
I would move on to a new phone!

As far as the band issue.
I could be wrong about this but I don't think it is possible to change the broad ban frequency your phone operates on.
At least I have never heard of anyone successful doing this!
Again, I would get a new phone!

Guttabkidme · Answer

You can do nothing about the freqs. I would not junk the phone. You have other options like flashing a custom rom or stock room.  Many tutorials to do so.

jfost · Answer

In doing a little research, I've been told accessing "Service Mode" directly on the phone makes it possible to change frequencies.? As I stated in the original post, this is all new to me so I really don't know if that is true or not.  I've also gone on to YouTube and watched a few videos showing it is possible, but I also know there's a lot of fake videos on there as well. If you both say it's not possible, then I'll go with what you say rather than a possibly fake video.
When it comes to the Custom Rom,  would that then give me phone access ? Or should I just move on to a different phone ?
I appreciate the assistance from all who respond and I appreciate the help thus far.