Hi Joe
Some comments below:
1. You must install the firmware that corresponds to the hardware version of the device you are using. If you try to install the wrong version (eg V1 firmware on a V3 device) you risk damaging the device.
Also, if the device is running the original manufacturere's firmware, you need to flash it with an OpenWrt/LEDE "factory" version of firmware.
You cannot use a "sysupgrade" firmware version for the initial flash. All VT SECN firmware is "sysupgrade" version.
2. The MR3420 V3 device is a more recent hardware release and as far as I can see it is not supported in OpenWrt or LEDE development.
So we are not able to develop SECN firmware for this version of the device.