@sandervg, Hi did you try setting the static ip from the actual LAN network instead of the client list? I cannot remember at this time if I had the same error. I might have, since I went with the method I am supplying below instead of using the client list.
LAN->Network Settings->VLAN 100? (I’m just guessing what your number or LAN is)->Under DHCP Reservation set the static LAN, with the MAC ID of the device you are trying to use.
See screenshot below.