I have a TP-link Archer A6(US) Ver. 3.0 with the factory software installed. I have downloaded openwrt-21.02.3-ramips-mt7621-tplink_archer-a6-v3-squashfs-factory.bin to begin using openwrt on this router.
When I select the option to upgrade the the firmware I get the message.
That is very odd... the two devices are not even remotely the same:
The A6 has 16MB flash and 128MB RAM, and a dual core MediaTek MT7621DAT 880MHz processor. The latest OpenWrt (21.02) easily runs on this device.
The 841N v14 has only 4MB flash and 32MB of RAM, and a single core MediaTek MT7628NN at 575MHz. It is not clear if it has ever been supported by OpenWrt, but can not be supported anymore because of the very limited hardware specs.
This sounds like a good idea. Go to TP-Link's website and download the latest version of firmware for this device... see if it works.
Was it sold as new or used? Was the box sealed?
Does the MAC address and/or serial number on the bottom label match anything on the device's web interface (assuming it is surfaced there)?
I managed to boot it to original tp-link software and it showed as Archer A6 accepted the openwrt installation and runs fine.
The bodgey RouterOS software installed was the problem. I presume this was installed by the Amazon seller. The router was bought new in an original box on Amazon with the RouterOS software installed.