Thank you very much for detailed explanation!
I did upgraded firmware. But it seams I’m in trouble now 
Before upgrading firmware I checked LAN_SDS_MODE and set it to 1 (it was 5 before)
# flash get LAN_SDS_MODE
LAN_SDS_MODE=5
# flash set LAN_SDS_MODE 1
LAN_SDS_MODE=1
After firmware update I was able to access WebUI / Telnet. And I decided to set LAN_SDS_MODE back to original 5.
After reboot I can not connect to my ISP, as well as access WebUI or telnet (
It shows some Tx when I set manual 2.5Gbps but with no access to ISP
Is there any way to navigate to WebUI, considering that it is running but with no access to ISP?
