I know this is a super old thread and maybe no one cares anymore, but I have hosed my 4G on two VZW M8 phones so far and read every post I could find on XDA about firmware flashing, RUUs etc., including this comment in the OP:
-----------------begin quote from OP
"HELP! I flashed this ROM, and now my LTE doesn't work...even on other ROMS!
First off, chill out. This is a known issue with HTC devices as a result of nothing more than flashing ROMS over and over and over and over again. While it is alarming, it's fairly easy (albeit involved) to fix. Read below:
Anyone having radio issues: The NV partition, which handles the radios, may be corrupted. Especially if you do a lot of ROM and firmware flashing. You can flash all the ROM's and firmware you like but it will not change the fact something is corrupt. The only way to reset the partition to out of the box stock is to flash your carrier's 4.4.4 firmware and the stock recovery for you device. Reboot to recovery and select factory data reset and let it do it's thing. If you watch the reset log you will see it mention resetting the NV partition. Then just flash back your recovery of choice and ROM of choice. This has been the proven fix for HTC devices for a few years now. "
----------------end quote from OP
I have flashed my Verizon stock firmware 4.4.4 and then got into stock recovery and did a full reset, the on-screen log showed Verizon NV getting reset, and then rebooted the stock rom and it DID NOT FIX MY 4G. I have then flashed every firmware that I could find down to the 1.55.605.2 version, which is the version my phone came with and the earliest firmware I thought was available, repeated the recovery reset and still NO 4G!
Just a few minutes ago I found the 1.12.605.11 RUU and flashed it (BTW, you MUST be S-off, or find an S-ON FUU version 1.12.605.11, if it even exists), and HOORAY!!! 4G is fixed, I didn't even have to do the recovery reset, just flashed the RUU zip file using hboot/fastboot method on my ext sd with the correct file name for my Verizon 0P6BIMG.zip.
I just wanted to post this because it may help others, it ended a months long frustrating nightmare for me.