Why does it not always respond to keyboard input when connected to a device via a com port?

Question

network hardware

the essence of the problem is as follows, I'm trying to flash the router.
connected to the router via COM port, configured everything. (57600, 8n1) displayed characters in the terminal are normal, but do not always respond to keyboard input (rx,tx,gnd connected).
sometimes the keyboard works and a strange character flies out (something like completely filled in, it's in PUTTY)
where to dig, I'll never know.
I connect via Arduino Uno (there is USB TTL 5v).

Answer 1

Swap rx-tx
Wrong :
RX <—> RX
TX <—> TX
Correct :
RX <—> TX
TX <—> RX

Answer 2

I'll try