Problems blocking IoT devices from internet (ASUS Merlin)

My home network consists of one router running asus merlin, and 2 access points running original asuswrt. I have disabled internet access on my IoT devices using the GUI, through the time scheduling option. However, my IoT devices frequently disconnect and try to reconnect to another AP. I figure this might be because they are trying to connect to the internet. This causes frequent disconnections and my IoT devices often do not respond. Is there a way to block their internet access and not have the problems with disconnections?