@SebAlvarez  What is your DHCP address range configured at?

The reservation must fall within the range.

  @DVair 

Some routers require it to be in range, some out of the range, and others can be either. 
 

Also seen when adding the MAC, use . Instead of : on a router. 
 

power cycle or reboot after adding it. 

  @SebAlvarez Same issue here, I have 2 assigned in the address range but cannot add any more. It seems to have started with the January 1.1.1 firmware. I tried to downgrade the firmware but it's not allowed.

Sorry, I was sure that I had replied to this thread with the solution. And it's been so long that I forgot exactly, but going through the router's config, I think it was this:

I believe that Static Routing was enabled, I can't remember why exactly. But once I disabled it, or deleted the entry that was there, then I was able to add the entries I wanted into Address Reservation, and it has worked flawlessly every since:

This is especially useful, even necessary, if you have a Mac with shares you want to access from the PC. With DHCP assigning different IPs on every computer boot, or router reboot, mounting Windows shares from macOS is very easy, but macOS shares from Windows is a pain, because as soon as the Mac is rebooted or whatever causes the router to assign a new IP, the network drive in Windows points to the old IP so it's useless. So doing the address reservation is a sure way to assign the same IP to the Mac every time no matter what.