Unfortunately the "Tunnel Network" field cannot be left blank and will always result in the server directive. This directive expands into:
push "topology [topology]"
if dev tun AND (topology == net30 OR topology == p2p):
ifconfig 10.8.0.1 10.8.0.2
ifconfig-pool 10.8.0.4 10.8.0.251
route 10.8.0.0 255.255.255.0
push "route 10.8.0.0 255.255.255.0"
else if topology == net30:
push "route 10.8.0.1"
if dev tap OR (dev tun AND topology == subnet):
ifconfig 10.8.0.1 255.255.255.0
ifconfig-pool 10.8.0.2 10.8.0.254 255.255.255.0
push "route-gateway 10.8.0.1"
Given your setup that would result in an ifconfig directive that wronly sets the ip address. As a workaround for p2p mode I have found the following solution:
-Add the ifconfig-noexec directive to the advanced settings.
-Assign your OpenVPN interface.
-Manually give it the correct static configuration.
Now, whenever OpenVPN starts, the wrongly generated ifconfig directive will no longer override your static settings. Et voila, you can configure it however you want. I do the same for routes. I remove any routes in OpenVPN itself, and just manually add routes to gateways set on the other side of my links. This also has the nice side effect of detecting a downed VPN by looking at the remote subnet's gateway status in the dashboard.
However, what I have not tackled yet is how to get this working in Remote Access mode. Apparently OpenVPN wrongly routes the .1 of my "Tunnel Network" despite my configured interface values. My guess is this happens because, although the OS is set correctly, OpenVPN itself doesn't know that the automatically assigned .1 server address is no longer in use. In p2p (site2site) setups this is no problem. It just always sends everything to the other side. However, in Remote Access mode (in OpenVPN it's called "server mode") OpenVPN itself needs to know to which client to route what data. Hence all the new iroute directives. Obviously, overriding the server interface does not override OpenVPN its internal routing and thus keeps believing it's the .1 in the Tunnel Network.
As the config file gets overridden on every reboot, I cannot see how we can currently use "topology mode" in combination with an alternative Tunnel Network server IP. Maybe someone knows how to use the field for extra directives to inhibit automatically configured directives? Or maybe we can prevent pfsense from overwriting a custom server1.conf file?
If not, a nice feature request would be another OpenVPN server mode called "custom". With no fields other than the "Advanced config" field. This way we would be able to do any complex setup while interface adjustments (precious dev-time) remain minimal. Devs?
Anyways, I hope this will help you. And let me know if you find other workarounds.