You got static IPs
Make sure the SMC into 'true static' mode, so it doesn't do NAT.
Log into the smcd3, and under Firewall/Options check both
Disable Firewall for True Static IP Subnet Only
Disable Gateway Smart Packet Detection
Under Port Configuration Check
Disable all Port Forwarding rules
Under 1-to-1 NAT check
Just for fun, make sure under LAN/Switch Controls
all ports are auto, the correct speed, duplex, active
Now the modem is more or less a dumb router, and you can assign static IPs to your devices behind it.
The only other possible problem is that you're assuming the GW is at the low end of your block - it could be at the other end.
With the /29 blocks at least, the gateway is the highest ip in the range
mine run from x.x.x.113-x.x.x.117, gatewaty is at x.x.x.118--
My place : »www.schettino.us