Under System > Advanced > Firewall & NAT > Network Address Translation there’s an option called “Enable automatic outbound NAT for Reflection”. Please try with this enabled. Also try both global NAT reflection options there, just to make sure.
Also, note that as an (in my opinion) better alternative to NAT reflection, you can use split-horizon DNS - if all webservers (namely, the proxy and the UNMS frontend) use ports 80 and 443. If you’re using pfSense’s DNS Resolver, simply add a Host Override at the bottom of the page:
Host: the subdomain (without the domain part)
Domain: the domain
IP address: IP address of the UNMS server