Hi
I'm facing the same problem at the moment, Have you find any way to fix it.
In fact, I talked with TP-Link support by email and they said "So sorry to tell you that you have killed it due to wrong firmware upgrade".

Thanks

Hello Aafzz1.

I am not impressed with the help that TP-Link has provided.
With the v4 firmware on the v3 the router will not even boot. I have confirmed that after the flash that the router hardware is perfectly fine.
You will have to open the router to get it to work again. You have to remove the winbond 25q32fvsig chip and flash it with a spi programmer.
I just cant find a flash dump for a v3. I have the router working with tl-mr3420 v1 firmware to test.

One thing I cant understand is why TP-Link does not add protection to the firmware for this. They can stop you from flashing the wrong model of router firmware so I cant see why they dont add version protection too.