eth0 = 192.168.0.5 default gateway 192.168.0.5 netmash 255.255.255.0
eth1 as below
If I configure my router with eth1 as 172.16.0.1 clients can access the INTERNET fine but cannot access their internal network on the 192.168.20.???? range.
If on the other hand I configure eth1 as 192.168.20.5 getting to their internal network works fine but they get no INTERNET.
The obvious answer is that their internal network gets configured to the 192.168.20.????? range but how do I then enable their INTERNET access ?
BTW A number of workstations have and need static addresses/IPs in the 192.168.20.????? range.

INTERNET modem <-> router on one network card/port eth0
router <-> hub <-> workstations on second network card eth1.

In router, the default gateway should be ISP IP address.
In workstation, the default gateway should be interface IP address of router on internal network.
If you provide route table in router and workstation, it will be much clear.