This question seems to be similar to others, but I'll take a shot anyway. A client recently switched ISPs from TDS to Comcast Business Class. Before the switch, they had 5 static IP addresses assigned. Now they'll have a single IP address that will change whenever Comcast decides to do so.

The issue is that this internet connection will be shared among two companies, both having (and wanting to keep) their own private subnets. Because TDS was supplying multiple IP addresses to the one location, this allowed me to put each router on the switch. Now, with Comcast, they only get one IP address, meaning there has to be a main router before the subnet routers. Luckily, the cable modem has a built-in router, which I would like to connect to each company's router, and still have DHCP enabled on all accounts.

Question: What do I need to do to the subnet routers to keep them separate from each other, but still allow internet access from the main router. I would love to say "I tried this", and give you links, but everything I find on the internet only mentions daisy-chaining routers with DCHP disabled.