You can use an ethernet adapter. But since the Mibox S only has USB 2.0, the maximum theoretical limit will be around 400 Mbps. It's still not good enough for bluray-quality 4K. But for streaming some compressed stuff from the likes of netflix or youtube it should work. To be honest I think the greatest weakness in the mibox S was to only have USB 2.0. With 3.0 this wouldn't be an issue at all.

It only has a single spatial stream which is 433mbps link rate (theoretical max with 100% signal) which is around 150 mbps in the TCP/UDP layer so yeah with strong enough signal you can stream 4k and I've succesfully did so with 4k HDR remuxes that were 80mbps and peaking over 100 sometimes. But ANY interference and it will be choppy and buffering. If you have plenty 5G networks around and any channels are overlapping with your router I'd be very carefull with picking bitrate of my remuxes.

I am getting speeds close to 120mbps

I get speeds up to 120

I run at 130mbps on 5ghz with my box s

I can't get much above 40Mpbs connected to my 5GHz enterprise access point with a strong signal (TP-Link EAP245). I'm waiting for my USB --> Ethernet adapter to come to see if it can get any faster and actually direct play a full Blu-ray rip through Plex.

200mbps no problem here on 5ghz wifi

Is my unit defective?

At home I get around 160mbps for both up and download.

Not sure if your signal is bad at where the mibox is but try to move it closer to your router or in a better position on your setup.

I'd use a wifi analyzer and optimize my network. Select manual router channels, and turn off any auto switching or AI stuff. Sometimes, adjusting ch bandwidth helps also.