Is device connected directly to a LAN port on ISP's router completed isolated from the subnet created by Sonicwall router connected behind the ISP's router.

Have small office. We have a Sonicwall connected to Verizon router. Verizons LAN IP is 192.168.1.1 and its DHCP issues IP range of 192.168.1.*. Sonicwall IP is 192.168.2.1 and its DHCP range of course is 192.168.2.*. We have a credit card machine that we've been told must be connected via VLAN on our network to be PCI Compliant. We figured if we just connected the card machine directly to the Verizon router rather than the Sonicwall that would isolate it from the internal network since they are different subnets but were told that this sort of configuration is not as isolating as a VLAN and thus not PCI Compiant. Is this the case?