RDP over VPN

[tdcockers]

Hi all, not sure where to put this so I'll start here. Apologies if it has been discussed previously, everything I've been able to find has been about issues connecting to RDP behind pfSense/VPN, not external to it.

My office is sitting behind pfSense with all traffic in and out via VPN (NordVPN). One of my clients has a remote desktop-enabled workstation directly connected to the internet (yes, I know that's a terrible idea, but that's what their trusted IT provider set up for them).

When my network traffic is routed though the VPN, I cannot connect to the RDP server, or (possibly related) send emails using their SMTP server. If I reconfigure pfSense to use the WAN as the gateway rather than the VPN, everything works normally.

I'm guessing it has something to do with the NAT rules, but I'm still quite new to any networking concepts beyond DHCP and DNS so looking for any sort of advice what to check next?