Hiya,
TFTP install from OS X, 10.15. Setup with the instructions here:
https://openwrt.org/toh/tp-link/archer_c2_ac750
IP - 192.168.0.66
Subnet - 255.255.255.0
Gateway - 192.168.0.1
Merged the stock boot and custom binary no problem.
Using official Apple USB > Ethernet adaptor. Model A1277. Tried with router Ethernet ports 1-4.
% tftp 192.168.0.66
(also tried sudo tftp)
tftp> binary
tftp> trace
Packet tracing on.
tftp> put final.bin
[plug in router, holding WPS button]
sent WRQ <file=final.bin, mode=octet>
sent WRQ <file=final.bin, mode=octet>
sent WRQ <file=final.bin, mode=octet>
received ACK <block=0>
sent DATA <block=1, 0 bytes>
received ACK <block=1>
I have tried with all the combinations of 10 & 100Mbps, full, half duplex. All same results.
set your Mac's ethernet port (the USB dongle is fine) in the system preferences > network control panel. Set it to manual and then set the address to 192.168.0.66 and subnet mask of 255.255.255.0 (you can leave the router and gateway fields empty).
copy the file to the correct location and start the tftp service on the mac
Boot your router into tftp recover mode (per the directions on the device info page) -- it will automatically download the file from the mac and then reboot.
turn off the tftp server sudo launchctl unload -F /System/Library/LaunchDaemons/tftp.plist
set your ethernet adapter on your mac back to automatic / DHCP (if you want it to work 'normally')