@marvosa
That actually was the problem. I was mistakenly connecting the router's WAN port to the LAN port of the Netgate. Admittedly I should have recognized the issue with having the two networks (Netgate - Router, then Router's LAN).
So, everything just about works. I want to access a particular host on the LAN (a network share). I looked up the DHCP client list and found its assigned IP address and was able to remotely (through the VPN) connect to it. Because I wan't this to be reliable, I assigned a static DHCP rule for this specific LAN host. But now the VPN client's cannot see it anymore. What could be going on? All other dynamically allocated DHCP slots remain reachable from the VPN.
I have a rule on the OpenVPN group to allow any to any, which is why the first part worked. But for some reason the statically assigned DHCP rule is acting as if it were not part of the LAN? I did notice that the host was marked as "offline" in the DHCP client list despite being active and reachable from other hosts on the LAN.
I tried adding a rule specifically allowing access to this static IP from the VPN, but of, course, the any to any rule takes precedence so this new rule does not get used.
Any ideas?
Actually, the issue seemed to have resolved itself after some time.