It took me more than two months, tons of reading and experiments to get my HTC One from Verizon (model HTC6525LVW, android 4.4.2.) to work in Argentina. Hope this helps others like me.
These are the problems I run into, in the next section I describe how I fixed them:
1.- Lack of power saving mode (making batteries run out in less than a day)
2.- Tethering (Mobile Internet Network sharing) not working
3.- No 4G access
This is how I fixed these issues:
1.- Installed JuiceDefender from Play.
You have to give it several days before it will actually produce results.
2.- To enable mobile network sharing:
2.1./ Obtain S-Off by paying u$s25 for Sunshine (download from Play). All free alternatives did nor work for me. If you are not S-Off, all changes are overwritten on restart.
2.2./ Obtain temporary root by installing KingRoot (download http://www.kingroot.net). I had to try several times until I got root access, but it is faster after the first time, and eventuallly I got it.
2.3./ Use RootBrowser (download from Play) to edit the file /system/customize/ACC/default.xml
Replace the 1 with a 0 in the following line
<item type=”integer” name=”TetheringGuard_rule”>1</item></app>
so that it reads
<item type=”integer” name=”TetheringGuard_rule”>0</item></app>
3.- To obtain 4G access
3.1./ Using RootBrowser, edit the file /system/build.prop
3.2./ Modify the property ro.telephony.default_network, set it to 9 (in my setup the property existed twice, I modified both entries).
9 stands for GSM+LTE, which is neither available in the settings menu nor using the Network or 4G Switcher tools.
Also, comment the following line by prefixing it with a #:
and add the following line:
The modified lines should look like this:
Restart and give it some time, when I did this I thought it had not worked until the next day I went out and found I had 4G. It seems I did not get good 4G reception where I was.
3.4./ You might also need to update/add an APN in you Settings > Mobile Data > GSM Options > Access Point Names
You can find the information for your carrier easily, here is one example (in Spanish):
Note: The Verizon HTC One M8 has 4G Band B4 enabled, so you should not need to do anything special to enable it).
These are some of the sources I used to compile this information, in most cases I am simply putting it together.