If you have an EA7300 v2 that is a known problem https://github.com/openwrt/openwrt/issues/11282 , also discussed USB is disconnected when WAN is up
There is pull request that is waiting for some minor improvement to updated terms it seems https://github.com/openwrt/openwrt/pull/15903
In the meantime the workaround from the above links is to run
echo 0 > /sys/class/leds/green:wan/brightness
You can add it as a line to your local startup script System->Startup then going to the Local Startup tab.
If you have an EA7300v1 then hard to say what the problem is.