I installed LGup and flashed it with the phillipines rom. My phone still says V10e-TWN-XX in the SW info but I have VoLTE support now, weird or what.
Anyone else wishing to give this a go will need to install Lg Bridge from the LG website which comes with the required USB drivers, then install LGup (LGUP_Store_Frame_Ver_1_14_3.msi) found here
https://www.mylgphones.com/download-lg-up-software. You will also need a DLL (LGUP_Common.dll) for the H990DS
https://drive.google.com/open?id=0B0-_MIhGeuKhNWFpcUNfc2FKeG8, copy this to C:\Program Files (x86)\LG Electronics\LGUP\model\common\ after installing LGup. if the folder does not exist, create it. replace the existing file with the one in the link. the one that comes with LGup will not work.
The regional versions of the firmware for the H990DS can be found here
https://lg-firmwares.com/lg-h990ds-firmwares/
I chose to upgrade in LGup, it retained all of my settings and I lost no data.
I'll test it out tomorrow and see what the VoLTE calls are like.