What version of MR3420 and OpenWRT are you running?
You have two ways to do it :
1. Install a direct firmware from the upgrade page which would go back to stock. This only works for a small number of devices.
2. Boot into safe mode and use telnet to transfer the firmware. This is sort of like using ODIN/Fastboot/ADB with network. This should work on most of the devices. TP Link devices have a safety mode inbuilt.