What is going on?
I have a bunch of timeouts every some equal time.
Router set as CPE on quick set. Firmware is 6.7
Give us more explane.
What is connect with that routerboard?
Have you select all the chains by HT?
On the second site is VDSL2 router which is working fine with other hardware.
During the time, I have had connected 3 other laptops to the same Wifi (on one of them, i have pinged the same addresses without any loss)
What do You mean by “Have you select all the chains by HT?” I am a beginner, so maybe I am doing some mistake 
Click on wireless interface, go to “HT”
See screenshot
I have RB951-2n, and i see only Chain0. I suppose, it means, i have only one chain
Yes, rb951-2n have only 1chain. What say log? Is there any error mesage?
I am not sure, which logs should i look at, but according to wireless and interfaces - totally nothing. Clear.
Left menu in winbox - Log
Go to “system” further to “logging”
By “topics” select “wireless”.
If this done click on the Arrow below and select “debug”
Click on “apply” and “ok”.
Look than on the logfile whats happend.
Right ping - on ethernet (no packet loss)
Left ping - wireless
During all the time i was pinging the router from other wifi device, no packet loss. So i assume, it’s not router making problems.
21:43:36 system,info log rule added by admin
21:43:44 radvd,debug skip Router Advertisement sending on bridge-local: no prefixe
s to send
21:45:01 radvd,debug skip Router Advertisement sending on ether2-master-local: no
prefixes to send
21:45:38 radvd,debug skip Router Advertisement sending on wlan1: no prefixes to se
nd
21:47:12 radvd,debug skip Router Advertisement sending on bridge2: no prefixes to
send
21:48:13 system,info,account user admin logged in from 192.168.1.167 via telnet