Failed to run golang program on AC-66U

[lotc]

Hi there,
I am using AC-66U with firmware 380.65_beta2.
Since my ISP only provides me a 10.x.x.x NAT IP, I try to expose my router to wan with some software.
Then I tried this one: https://github.com/fatedier/frp

While I tried to run it on my AC-66U over a terminal.
It showed

 connect to server [123.123.123.123:123] error, dial tcp 123.123.123.123:123: errno -9

(I changed the server IP and port for privacy)

I googled for hours but find nothing mentioned "errno -9".
Is there something wrong with my iptables or other settings?