ISPLAN y.y.y.y/26 are real public IPs. So you just want pfSense to route those, and not do NAT.
Firewall->NAT, Outbound - switch to Manual and delete the NAT rules for that y.y.y.y/26 interface.
For initial setup and testing put:
pass rule on WAN to allow source any, destination ISPLANnet
pass rule on ISPLAN allow source ISPLANnet destination any
Now put a test device in ISPLAN, you should be able to get out from it to the internet.
The ISP should be routing anything for y.y.y.y/26 to your public WAN IP, so get on the real internet and try accessing that ISPLAN test device. It should be reachable.
Once you know the routing is working fine, then put more restrictive rules on WAN and ISPLAN to allow only what you really want, and setup the real servers…