I'm still trying to figure out my network settings so that I can have my IoT one one network while still being able to access my home assistant from the other network.
Unfortunately, my ISP is also my cable company, and I have to use their modem/router combo else the cable boxes won't accept the cable signal. I'm using my own wireless access point (which also doubles as a switch for the handful of Ethernet devices I have), and it can split off a separate SSID, but that's not really doing much.