Hello, hopefully this doesn't break the rules. I couldn't find any posts of people actually managing to fix their Huion, so I decided that this might give people somewhere to look once they tried everything else.

Disclaimer and prerequisites

1. Make sure the "tablet" part of your huion is actually not working. It should not make any "device connected" sound when you plug it in.
2. Try all the other methods of fixing it first. Make sure it's not user error, try uninstalling / reinstalling the drivers, try different drivers, try no drivers, test on a different PC, try with a different pen, try using the 3-in-1 cable vs try the 2 usb cable method (see manual), try both orientations for the USB-C cable(s), use your warranty if you have it, do literally everything possible before you try this.
3. You will need to know how to solder.

** Again, do this at your own risk **. I'm writing this as a place to start looking for people who would consider themselves handy enough to do the work, since apparently literally nobody has tried to fix / take apart a drawing tablet, even though a lot of people seem to have this issue.

The actual fix:

I fixed my Kamvas 22 plus, but this should work for any of the display tablets huion makes, though the steps may not be the exact same.

1. Take apart the tablet (this voids the warranty obviously).

To do this, flip the tablet over and remove all the screws on the back. Take care to not damage your screen while you work on the tablet. There will be 1 screw that has a tamper-proof sticker over it, and there will be a hidden screw by the USB ports, underneath the serial number (thanks huion). There were no screws under the rubber feet on the Kamvas 22 plus at least. There were 8 large screws and then the 1 tiny screw hidden under the serial number on my 22 plus.

Next, you will need to pry the casing open, this is quite annoying and difficult, and will likely leave some marks on the back housing if you are not careful. To do this, use a metal pry tool (or knife, but less ideal), and start pushing the tabs in. There are a dozen tabs on each side of the tablet, located on the screen side, (so you will pry the back housing off by pushing in, and then prying the back casing up). Once you get started, it should become easier to pop all the tabs off.

You should now see something like this: https://i.imgur.com/el0q62U.png

2. Diagnosing the issue

From the image above, the board on the side seems to be the main controller / hub for the display tablet, the bottom seems to be the actual LCD display (based on the parts number), and then the right board seems to be the "tablet" part of the tablet.

First do a quick inspection for any shorts, (ie if you ever spilled something on your huion, that could be the culprit!). In my case, it was squeaky clean.

If you have a multimeter, a good thing to check if you are getting power on the 3.3v and 1.8v test pads on the tablet board. You will need to plug your huion in, then measure the voltage between each of the pads and ground. If the reading is 3.3~ and 1.8~ respectively, that means you likely have the same issue as me.

In my case, it seemed like there was something wrong with the "main" board (possibly with the USB hub, but I didn't have the expertise to diagnose further). Since we are getting power to the tablet side, that means that part should still be working.

3. The fix

One thing that you'll also notice is that the cable connecting the "tablet" to the "main" board looks awfully a lot like a USB cable, which makes a lot of sense. If you know a little bit about how these tablets work, they are quite literally just a screen over a traditional drawing tablet. (you can look up DIY drawing tablets to get some more details).

What I ended up doing was splicing a USB cable to the cable that attaches to the "tablet" board, then I drilled a hole in the housing to run that USB cable separately to my PC. https://i.imgur.com/UCmcOSc.png

Just plugging this USB cable in alone to your PC, you should now hear the "device connected" sound, and your tablet should actually control your mouse, even without power / display.

Now all you need to do is put it back together, and you will be able to use your tablet with the 3-in-1 cable + this extra spliced cable for the tablet.

There's probably a much nicer / cleaner way to do this, but since I couldn't find a single post actually solving the issue (other than just buying a new tablet lol), I thought I'd create this so that at least people have a reference.