Hardwire everything you can over ethernet to get them off Wifi.

Use a dedicated 2.4ghz AP for all IoT devices. Firewall this network and only allow the traffic those devices need. This greatly reduces congestion.

Use 5ghz for phones/laptops and keep IoT off that network.

That's really about it. If you have special circumstances there are other solutions, but generally the solution to bad wifi is to not use the wifi, lol.