@jcyr using a /25 vs a /24 is not a "hack" hehehe
But to be honest use of 192.168.1 or even say 192.168.0 can be problematic - these are 2 of the most common networks. So you run into stuff like your seeing with your ATT devices IP address. You can also see issues with remote access via vpn where the remote site is also using the same IP range your using and then have problems accessing stuff via the vpn.
Using the 192.168.1 could also lead to problem if you fire up some new device on your network and it defaults to using say .1 or .254 and that ends up stepping on say pfsense IP..
You were better off just using 192.168.2 network, or if you like the 1 there as the 3rd octet.. Use maybe 172.16.1/24 or 10.1.1/24 or 192.168.10/24
But using a /25 can work.. for your current setup, but could also be problematic because I doubt your ATT device is using a /25 and you could still run into a issue. If it happened to say assign your pfsense wan the same IP in your /25 as your lan side network interface..
Your better off just moving to some network on your pfsense lan that in no way is overlapping with the network your ATT device is using.