Probably a hardware issue, or bad soldered wifi antenna.
That depends: as the OP is describing it actually sounds like he switches the wifi switch to "On" and instantly (or after a few seconds) it turns off by itself. That makes me doubt that it is an issue with the soldering of the antenna. It's probably related to the WiFi chip itself.( so probably not an easy fix)
Ah maybe worth opening the guts and having a poke
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I wouldn't open it up. The guts of the XD are kinda fragile and it's easy to break something.
If you still got warranty send it back to the retailer/GPD.
with OTG i dont see why not. I guess it depends on drivers n stuff. I'm not to savy on it.
Saw this video: https://www.youtube.com/watch?v=fu0ixxeYemc&t=53s
Like skelton already said the linux kernel used is "3.10" from 2013 It is used with most android devices probably for stability and compatibility reasons. So there is a good chance that dongles from 2013 or older ones will work as long as they work with open source drivers. I remember that back in the day (around 2008 when i still used wifi dongles with my desktop) a lot of wifi dongle chipsets used proprietary drivers or proprietary binary blobs making it close to impossible getting them running with linux.
So i'd suggest checking which wifi Chipsets have open source drivers inside of the 3.10 kernel and then buying a stick with one of those older chipsets.