Knowing nothing about your ST780WL, and, that your environment is configured with NAT before reaching the OpenBSD box -- you will not normally see incoming packets to your public Internet IP address on your OpenBSD platform, unless a state has been previously established -from- your OpenBSD platform.
NAT routers can be set to forward inbound traffic to specific UDP and TCP ports, or forward specific protocols that do not have ports, and some can also forward all traffic, using a feature typically called "DMZ", though the name and capabilities offered vary from one NAT device to another, depending on vendor.
Otherwise, all you've proved is that simple stateful NAT, offered by thousands of diferent off-the-shelf devices, provides intrinsic "firewall"-like capability, which is one of its basic value propositions.
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