I have a 2860ac on a fixed IP address but cannot access it over the internet from another PC when I type the IP address in the browser.

I have "Allow management from the Internet" checked in System maintenance/Management.

"Domain name allowed" left blank. Should I have something for this?

Thanks

I have a 2860ac on a fixed IP address

You're absolutely sure it's routable/public ? ( No CGN etc.) .
You don't need a domain entry.
Have you tried it from elsewhere - i.e. from outside the same network (try from your phone on mobile data, not wifi)