Subnet Mask

Hello All, I am a novice when it comes to networking what I think is a
very basic question.

I have a DSL modem with a preassigned internal IP address of
192.168.0.1. I want to add a hardware firewall behind the DLS modem
and it has a preassigned IP address of 192.168.1.1. Since the 3rd
Octet is different (one is a zero the other a one) I’m assuming I
can’t use the subnet mask of 255.255.255.0 and must use the subnet
mask of 255.255.0.0 for the two devices to see each other. Is this
correct?

Thanks.
Gi