All my radios work fine from the start, just needed to configure them with Luci and the rest is history. Fixed several issues in the process.
My first post here :
Depending on retailer, you might not need to prove anything except saying you have intermittent connection issues... Just make up a story and make sure you reflash to double stock FW beforehand.
Bestbuy, Amazon, Staples, are great with returns. Just return it and once refunded, just order another one...
I have assembled thousands of clone PC'S and managed several networks in my young days and have encountered countless weird issues, waiting countless hours with software, just to end up fixing with hardware replacement, either parts or entire units often with same model and make.
If hardware don't work the was it's supposed to with fresh (emphasis fresh, like in you case) production software, then it's a hardware issue. You can try and fight it as much as you want, but in the end, it's supposed to work the first time when using supported hardware and software revisions as in this case. Also, limit yourself to production versions like 19.07.3 and not untested snapshots. If it don't work with production, it's defective...
Once you have a known working state with basic install, you can go on and experiment all you want, knowing you are one flash away back to known working state if you put yourself in trouble.
Mine worked from the very first time as I would expect it to. I had researched the EA8300 here, prior to buying in order to make sure I had the confidence this would work.
I have yet to use the command line to configure anything, as I am happy with the result using LuCi for now. Hopefully, I can get my other MR8300 upgraded eventually, as it's essentially an EA8300 with 512Mb ram...
My advice, return it and buy another one.
Just my two bits as a old dinosaur who survived a few meteor showers...