@Efonne:
If you want access through WAN on both subnets, you will need to add another allow rule on LAN for the other subnet and I think you need to manually add another outbound NAT rule for the other subnet (change to manual outbound NAT).
Exactly! It took a few minutes to get it working but it works great now. Just for future reference (so I can remember), to add a secondary/additional IP for the LAN segment:
Click Firewall–>VIPS and create a new IP Alias for the gateway IP Address (ie: 172.16.155.1/24)
Click Firewall-->Rules-->LAN and add the new subnet (Proto: *; Source 172.16.155.0/24; Port *; Destination *; Gateway *)
Click Firewall-->NAT-->Outbound NAT and enable Manual Outbound NAT. Click Save/Apply. Then, add a new NAT rule using Interface: WAN; Source 172.16.155.0/24; Source Port *; Destination *; NAT Address *; NAT Port *; Static Port: NO
Thanks to everyone for helping get this working!
-Ron