If it's any help I use a NetBSD as my gateway, I have two RealTek 8139 cards, as rtk0 and rtk1,

my rtk0 is on the same subnet as my ADSL router

my rtk1 is on my internal subnet

consider each subnet separately, what address and subnets do they need. Confirm the connectivity on each subnet separately.

Next, sort out routing, I enable IP routing and set my default gateway to point to the ADSL router.

Then, if you wish, sort out a local DNS server on the gateway.

And finally add a DHCPD server to provide local IP addresses on your internal subnet for DHCP enabled machines.