Can't get Serial Output

I am new to OpenWRT and I've tried to build my own image.
My Router is ZIO-FreeZio and it officially supports OpenWRT, but the version was too old to use.(It used v.14)
So I decided to build my own firmware using this repo
But I just missed select target, And I've just flashed it.
After that, It stops blinking LAN LED & WAN LED.
Only Power LED shows up....
So I decided to recover my router using Serial, but nothing shows.
what should I do?

Tried swapping the Rx and Tx cables?

If you short the Rx and Tx pins, can you see what you're typing in the terminal app ?