Multiple networks

Hi All, This is my first time reaching out on this forum. To try to make a long story short, they have on IP address they get from their ISP. The gateway leased from the ISP has onboard/built in WIFI. They also have a 3rd party firewall that does all DHCP/NAT/Routing for the internal network. We put the gateway into Bridge Mode to have the interface on the firewall get assigned the Ip from the ISP. After doing that, I found I can still connect to the gateway built in WIFI network and browse the internet, even though the IP has been assigned to the firewall. Can anyone help me here on if I'm missing something? If the gateway is no longer routing or doing DHCP, I'm wondering how the WIFI network is able to connect to the internet, when a completely different network should be the network connecting to the internet? Thanks,