I have purchased numerous GL-MT300A and M devices for configuration and selling on to my customers and I have not had any problems so far.
New and straight out of the box it should come up as a router with the lan port and wifi running dhcp on the 192.168.8.x subnet.
If “Out of the Box” this is not the case then most likely you have a rare faulty device.
In my own development I bricked and recovered many times. Remember you must connect your pc to either port but set your pc to the static address of 192.168.1.2 otherwise the uboot ui it will not work.