I can't speak for the GL.inet devices, power is going to be your biggest woe, but I have used both Y connectors as Belkin PD powered Ethernet, I've had luck with both but the former (Y splitter in conjunction with RPI) seemed to have conked out over time. And the ability to power the device is also very inconsistent. Since you're trying to connect it to WAN port, I recommend the Belkin PD Powered Ethernet device. And you'll want a nice power supply for the type-c PD port on it. I'm using a 65W GaN charger. The charger that came with M2000 is inadequate.

I have the Belkin ethernet connector connected to the WAN port of a router that acts as another gateway device in my existing network.