Hello,
External devices may not be recognized for a few different reasons, let's try some troubleshooting to narrow things down.
Firstly, have you installed the most recent Windows Updates? If not, that's always a good first step in solving Surface issues. You can find more info on how to ensure you have the most recent updates here: https://support.microsoft.com/en-us/help/402350...
Once you ensure you have the latest updates, try a two-button shutdown with the keyboard and mouse attached. I assume you've done this because you said you did a full reset, but sometimes giving it another try can help the issue. If you're not sure if you did the full two-button shutdown, here's the steps: https://support.microsoft.com/en-us/help/403628...
Once you do these, let me know how it goes. If they don't solve the problem there's more we can do to get this taken care of. If there's anything else I can be doing to best assist you just let me know. Hope this helps!