@djwujek What is your current firmware version? Can you send a screenshot of the error code you're getting? 

  @NeilR_M 

  @djwujek 

I just checked it on another router, but it's the same with v1. The update won't work and I get an error that the update file isn't valid. I don't know if this is because the beta version was previously installed or if the file is actually bad. Could someone from TP-Link verify this? There are serious vulnerabilities and it needs to be updated as soon as possible.

  @djwujek The beta version will be your issue here. You'll need to downgrade to a release version first, and then to the patched version. 
Another user had the same issue with a different model here: URGENT: ER8411 firmware issues after exploit - Business Community

  @djwujek 

your only options are Emergency mode downgrade, or (easier) SSH roll-back, then you can update to release version

  @djwujek 

log in to the router with SSH, use device username and password. once logged in you must first enable, the commands are in this order

enable and press enter, then
roll-back and enter to roll back to previous version.

you can also use ? for more info in each menu

  @MR.S 

Can this be done from the controller level or is it better to reset the router to factory settings, log in to it, enable ssh and only then issue this command?