The DHCP range that Open-Mesh APs use per SSID are not directly configurable. If you want to control the IP range on just one of the SSIDs, the easiest way to do this would be to bridge the SSID to your LAN so that this SSID will use the DHCP range from your router, and you can control the IP range from there.
If you wish to control the IP range of more than one SSID however, you will need to configure your LAN equipment for the desired VLAN settings, and associate each SSID to the desired VLAN using the VLAN tag fields. Note that you will need to be using manageable network equipment that allows for VLANs to use this feature.
SSIDs can be bridged to the LAN using the "Bridge to LAN" toggle on the SSID settings page, or they can be bridged to a VLAN by entering a VLAN ID in the "Bridge to VLAN" box.