You cannot remove a temporary ban within the software. You can only remove a permanent ban. This is silly. I dont know why this is the case. If I recall correctly on earlier releases all bans were placed in the same list in the webserver advanced settings. Easiest thing to do would be to reserve a new ip in your router for the banned device. You can email support with this issue so it can be resolved in the future.
From the help file
If an address is denied access on this list it is considered permanently banned. There is
actually a second temporary “denied” list maintained by the software that is not visible here.
If you choose to auto-ban an address after a specific number of failed login attempts, that
address will be added to one of these lists. If you select the option to release the ban after a
number of minutes, the banned IP address is added to the internal temporary list instead of
the one visible and editable here.