Sorry bumping up this old thread.
As far as I being informed, we can't have internet access by connecting PC to the BTU Huawei (optical modem) ethernet ports unless we connect to the wifi router LAN.
I have similar situation where I need to shift my wifi router to upstairs through a long LAN cable that connected to BTU Huawei.
This is because I want to cover the upstairs wifi signal temporary. In fact, I can have balance wifi signal for both upstair and downstairs after I shift wifi router to upstairs.
I know you would suggest wifi extender / powerline but I need some time since we are having MCO now...

I wish I can connect my downstairs desktop directly to BTU Huawei through LAN cable.
So, this method above would work, while maintaining the same working condition for upstairs wifi router?