If you have the MR6400 still, can you open it up and confirm if its running a Mediatek chip and if so what model? You'll see the numbers printed on the chip.
I have tried to load openwrt on the MR6400 v3 and found its the MT7628, not Atheros like the v1. I don't know what the v2 is but suspect its probably MT7628 also. There is no support for openwrt for the v3 either, so I started looking at the boot loader via UART cable.
The flash on the v3 is 8mb, ram is 64mb. The kernel starts at offset 0x20000, and before this is the U-Boot partition.
So I started looking at adding support. Investigating existing TP-Link devices supported by openwrt, the tl-mr3420-v5 looks to be exactly the same hardware including the same flash layout.
I next tftp'd openwrt-ramips-mt76x8-tplink_tl-mr3420-v5-squashfs-tftp-recovery.bin and got the MR6400 v3 booted from this, ethernet and wifi working fine, although the GPIO's are all mixed up causing the LED's to behave erratically or not at all.
I haven't tried going any further with this yet, the next step will be to sort out the GPIO's and see if the 4G module can be accessed.
tftp on the MR6400 v3 looks for tp_recovery.bin at 192.168.0.225