Thank you for responding.
Is your router dual-band?
Yes, and 802.11 ac
What may be happening is that your GT King is connected to the 2.4GHz band on your router instead of the 5.0GHz band. This would result in slower speeds.
The Beelink can connect to either, but the speed of the 2.4 GHz connection is very slow (sometimes as slow as 6 Mbps) and the 5 Ghz is the one that is 50-82 Mbps.
Some routers make use of a "Smart connect" system which concatenates both bands into a single connection - and the router does the behind-the-scenes work to give you the optimum speeds.
This isn't perfect however, as few machines are.
I have an Arris Model TG02GHQ2472GP2 modem/router and the documentation doesn’t mention any “Smart connect”. It says the 2.4 is on channel 11 and the 5.0 is on channel 36.
Also ensure that the GT King Pro is located in the same place as the Onn box.
They are both under the TV about a foot and a half apart.