Hi there!!!
I have Tplink wr-840nv3 router, Its a great one for simple use like for extender or for simple low usage router but unfortunately its only a 4MiB flash and 32 MiB of Ram and doesn't have a USB port. Back in the day I was reading some FAQ's about this router I found that somebody had added a USB port(solder on board) but he didn't wrote how did he added to it he wrote that he will explain it but its about 3-4 years no response(Now I was not able to find that page).So I am wondering if someone know how to do it just make some Schematic over the board pic or make a video about it or something like that, it will be really helpful!!!
Please consider it if you can do it
Thanks in advance
Tech data of router(https://wikidevi.com/wiki/TP-LINK_TL-WR840N_v3)
By the way I am running latest OpenWrt 18.06.1 r7258-5eb055306f