If you can't use Bonjour to discover (browse/access) your Filemaker server, it's probably because your base station is running NAT, which blocks multicasts between [W]LAN and the WAN port. Having NAT enabled is also why the AirPort Utility is restricting what DHCP ranges you can use.
If you've already got a DHCP server on your network serving 172.20.x.x addresses, then put the base station in bridge mode and let your iPad get a DHCP lease from your existing DHCP server (this is probably the best case). You probably already have a NAT on your network as well, so turning off NAT on the AirPort base station is good since it avoids double-NAT, which can be problematic.
If you don't already have a DHCP server on your network and you want the base station to just be your network's DHCP server and not its NAT gateway, then switch it to "Share a range of IP addresses" (this is Apple-speak for "NAT service off, DHCP service on"). Once you do that, it should let you put in any reasonable address range for the DHCP server. It might warn you that using RFC 1918 private addresses might not be what you really want to do in that mode, but I think it lets you ignore the warning and it works.