RT-AC68U port light, help

[NellyJelly]

Hi, Hope i can get some help, I have a AC68U as my main router plus one more set up as a AP. I have a 3 floor house and main router is on bottom floor, AP on 3rd floor connected by ethernet. I was running Melins firmware on both, until internet connection kept dropping. In the end internet became very slow until i unplugged the AP. I saw at LAN port 1 was lit without anything being plugged in it. I tried to run firmware restoration program but it stalls. I noticed the reason was when it was restoring Lan port 1 would light during the process which caused the restoration to fail. I managed to connect to the router, restore and set it up as a repeater. It works fine for a day and then crashes my net.
My question is what is happening, why does this lan port light up when nothing is connected to it. Why does it crash my net and is there any way to fix it.. Really grateful for any suggestions..