Can USG110 handle more than 254 DHCP Leases

We have a USG110 set up with a VLAN with DHCP enabled, start address 10.10.10.2 and pool size 250. This is connected to a smart switch which feeds a number of wifi access points. The problem is that even with a low expiry time, we are running out of leases for what are almost all mobile devices. I sort of understand the concepts and I think the options are either change the subnet to allow more addresses on the 10.10... subnet or add another vlan with a different subnet, although I don't know if this works with a single SSID across the different subnets. How should I tackle this? Thanks.