ESP8266 AT Commands: Where to Find Full Error Code and UNLINK Error Documentation?

I have searched for full documentation for the ESP-01 but to no avail. I managed to collect command descriptions, examples but I don't have a full description of AT commands with returned errors.

Once every few days, querying the NTP server results in an UNLINK error. Nowhere have I found a description of the error, I presume there is no response from NTP. I have selected another server, it responds to pings, error same. I haven't yet fired up Wireshark to see what's going on in the network. I'll have to, then I'll take a look to see if a response is coming in.

I'm sending the query by address, not name, so the DNS problem is ruled out.
Other ESP functionality is working (it is a web server).

Where can I find full documentation of the Chinaman in 'normal' language?

Everything is described in the documentation below.

Thanks a lot.
I have now found out that with AT commands I can control the GPIO pin. I will probably find out a lot more.

EDIT .
My euphoria was premature. I could find nothing about ULINK. The module answers:
.
But in Wireshark I have only:
.
MAC query when sending the request. The conclusion is that the frame is not coming out of the ESP.
How about resetting the router?