New Firewall won't take old IP Scheme

I am upgrading a Watchguard firewall for a customer and I can't get traffic through when I use their existing IP setup. The current DHCP scope is:

198.249.148.x

When I use this, I get no traffic to the network. If I use a standard 10.0.1.x traffic passes fine. I don't see any special setting in the old firewall that would cause this to work so I thought I could just copy over the settings from old to new. Is 198.249.148.x an invalid scheme? I am just confused because it works on the old firewall.