Terminology: a static IP is called DHCP Reservation
You do not need to establish a new subnet.
Static IPs can be assigned in both the untagged LAN and/or in a VLAN
Clickpath in firmware 7: Network -> Network Settings. In the LAN section, click on either the untagged LAN or a VLAN. In the LAN settings, DHCP reservation is at the bottom. If the device has recently connected, you can get the MAC address from the Client List and copy/paste it into the DHCP reservation.
One wrinkle to be aware of is that Windows 10 and Android (and maybe other OSs too) are now starting to randomize MAC addresses.