1. WAN – config as usual with your @unifi username and password using PPPOE
2. Log into pfsense shell:
   • cp /var/etc/mpd_wan.conf /conf
   • edit mpd_wan.conf and replace ip1-ip3 to your public IP and save the the file.
     • set iface addrs !ip1 !ip2 !ip3
   • Reboot or restart WAN interface
3. Adding IP at Firewall – Virtual IPs
4. Adding NAT 1:1 corresponding to the local IP server.

Done.

Thanks to – https://forum.netgate.com/topic/42981/static-ip-on-pppoe-wan-connection/5