Installing OpenWrt on Archer C2600 v1.1

[bomberb17]

Just received a refurbished TP-Link Archer C2600 from Newegg and want to flash OpenWrt.
My laptop has Ubuntu 20.04 and is connected directly to LAN1 of the router. What I am doing is the following:

1. Install tftpd-hpa server
2. Download openwrt-21.02.1-ipq806x-generic-tplink_c2600-squashfs-factory.bin, rename it to ArcherC2600_1.0_tp_recovery.bin and copy it into /srv/tftp directory (which is specified as TFTP_DIRECTORY in /etc/default/tftpd-hpa )
3. Set laptop IP to 192.168.0.66, subnet mask 255.255.255.0
4. Test the server:

tftp 192.168.0.66
tftp> get ArcherC2600_1.0_tp_recovery.bin
tftp> quit

5. Keep reset button on C2600 pushed, power it on, keep reset pushed and wait for ~30 seconds. LEDs are blinking (have no idea what's happening) and then router reboots.

Unfortunately, after this process completes, the router still has the TP-Link factory firmware.
Any ideas?

[bomberb17]

Here is also output from wireshark. Everything looks normal, but openwrt just won't install..

[bomberb17]

Switched from Ubuntu to Windows and successfully installed with tftpd64.
Not sure why Ubuntu didn't work though.