Hi @dragonslayr
The default setting for IP’s for the OpenVPN clients is dynamic. If you want to apply firewall rules to OpenVPN clients you need to set up the Static Address Pools and you can then assign a fixed IP address for each OpenVPN client.
On the Global Settings section of the OpenVPN page press the Static IP address pools button (turn off the openvpn server first) and follow the wiki page
https://wiki.ipfire.org/configuration/services/openvpn/config/static_ip
Once you have set up the Static IP address pool then you need to edit each client and the static IP radio button will now be present as @tphz showed it and you can then define the static IP for each client that needs it.
You will then need to re-install the conf file for each client that you have changed to a static IP