If your prefix is always skipping, you have to complain to your ISP and meanwhile automate your network renumbering. Is there a reason you are not using the pfsense device to do routing and (put the cpe device to bridge mode)? Then you don't need to worry about your firewall not knowing about reboots, and the firewall rules will just work (assuming pfsense supports using prefix relative rules, vs hardcoding the prefix to all firewall rules...)

Or mDNS: http://www.multicastdns.org/

As used by Apple’s Bonjour implementation of the Zeroconf networking stack.

If using Debian or the like, just apt install avahi-daemon and libnss-mdns, and access your hosts on the local network as servername.local.

Yes, you would probably then have to allow inbound traffic to these addresses in your ISP router's firewall or whatever you have.

If your router's default configuration is sane it should (by default) only allow returning connections and drop any other.