Update: SOLUTION FOUND
TLDR: Ethernet cable going from my PC and into a wifi range extender has completely resolved the “Connecting Failed (0x00020009)” error.
Well, thankfully, after 1 month I managed to discover a solution! I kept wondering why connecting to dayz via wifi worked on my laptop and not on my PC…. Naturally, I assumed it couldn’t be a hardware issue with my PC’s wifi card since I have flawless wifi on my PC and don’t have connection problems with other games AT ALL. So, I was thinking that somehow the game files had got messed up and was preventing the connection that way. I was WRONG. It turns out that, for whatever reason, dayz specifically doesn’t like my PCs wifi card (Gigabyte GC-WB1733D-I) but is totally fine with my laptop’s wifi card. I was loath to spend money on purchasing a new wifi card for my PC (one that dayz might not accept anyway) and I couldn’t connect my PC to my router with an ethernet cable since my router is downstairs and PC is upstairs - and I don’t want a really long cable messing up the home. So I was stuck.
Buuuut, I eventually came across a device called a ‘wifi range extender’ which has an ethernet port in it. So I thought that it might be worth a shot to try and see if that would solve the problem. All I did was plug it into a socket/electrical outlet near my PC, connect the extender wirelessly to my router via WPS, and then take a 1m ethernet cable connecting the wifi extender to my PC. Lo and behold, it solved the 0x00020009 error! No mess and no long cables - just a small device plugged into a nearby socket/electrical outlet. So DayZ is working fine for me on my PC now and I am really happy 😄
The specific wifi extender I am using is this model here: https://www.amazon.co.uk/gp/product/B0993XJ1G4/
Just as a side note, a few weeks ago, I discovered a partial solution to the 0x00020009 error which was far from ideal. It turns out that if you just keep trying to connect to the server, it will eventually let you in. Although you can be sat there trying to get in the server anywhere from 16 seconds up to 10 minutes (one time it took 16 minutes :s) - with it usually taking 2-3 minutes of error messages before eventually letting you in. And this must be repeated every time you want to go back on the server. So, yeah that was really annoying and tedious. Thankfully, ethernet into wifi range extender solves this 😄