S4 root to fix 4G/LTE issue?

I'm running stock 4.4.2 on my S4, but I can't get 4G/LTE connectivity. When I power it on, it shows 4G for 30 seconds and then goes to H or 3G. Of course I tried my sim with another phone and another sim in my phone, and they all work.

Now I want to do this:

Quote:

After you have rooted the device completely, go to Google Play and install 'ES file explorer'. Open the app, and press the menu key on the device and you will get a panel to your left on the screen. Scroll down and enable Rootexplorer (you will ask to grant permission by the SuperSU app since you have rooted, so give permission). Now use the search function and find the file with this EXACT name 'persist.radio.lteon' and delete it. Please make sure you are deleting this exact file. Don't delete anything else and end up bricking your device. After deleting restart the phone and that's it! You will have the LTE/WCDMA/GSM option in the network mode once you are booted and you will receive 4G signals if you are in an area with LTE coverage.

I just want to root my phone, do the above and then unroot. What app or method is the easiest to do that?