Allow Subnet Mask of 255.255.0.0 and Flexible DHCP Start (Orbi AC3000/RBR50)

My Orbi AC3000/RBR50 has the first three bytes of the subnet mask greyed out and preset to 255.  That means my subnet mask can be only 255.255.255.0 to 255.255.255.252.   My suggestion is to allow ...