Black Ops 3 - Friends Unable to Join My Lobby

I have played Call of Duty: Black Ops 3 (BO3) Zombies with friends before on Windows 10.

I’ve been playing with friends on BO3 Zombies recently and one would always host.
Trouble comes in when anyone else tries to host, even myself.
I would like to resolve possible issues on my end to be sure.

I suspect my NAT Type may be the issue.
I’ve seen in BO3’s Menu>Options>Network, there is an entry for “Nat Type” it suggests changes to enact to change the NAT Type to “Open”

I am very sure that I have port forwarded port number 27017 as stated in the entry.
I have enabled UPnP within my router and added my computer’s local IP address in the DMZ.
After making sure all of those settings are correct, BO3 still lists my NAT Type as “Moderate”.

Could there be steps needed to be performed in my operating system to change the NAT Type to “Open” or to make it easier for my friends to connect to my lobby?