networking

Hey all, I was wondering if anyone could help me work out how to do this? Basically, I have a stupid number of smart devices and my router has become increasingly unstable. I want to have all my IOT devices on one router and reserve the other for priority devices like phones and PCs.

I plan to put my IOT hub on 2G only and my primary hub on 6G and 6e only to avoid 2G congestion.

Problem is, if I connect both my routers to my modem, only one can connect to the internet. I tried putting a network switch between the routers and the modem, no dice.

Does anybody know how I can have 2 separate networks using 2 separate routers on a single modem? Both require internet connection but they don’t need to be able to communicate.

Thanks in advance for any help people can give :)