@lagreca:
On this end, I can ping a remote LAN machine using the Diagnostics -> ping functionality.
If you do that pfSense uses the VPN IP, which is known by the Asus router, of course.
If you cannot add a static route to the router, you can also solve this by NAT.
Go to Firewall > NAT > Outbound, if it do automatic rule generation, check hybrid or manual and hit save.
Add a new rule:
Interface: OpenVPN
Source: Network and enter your LAN network
The rest can be left at defaults, save it.
If you have more than one OpenVPN connections, you have to assign an interface to each at first and use this in the rule here.