@Prof 

Just want to confirm, have you tried to reset the router to factory defaults after the firmware update? Give it a go if the 2.4G is not stable even after you modified the wireless settings, such as wireless channel and channel width.

How about the 5G and wired connection after the firmware update, are they still stable?

@Prof 

Thank you for all the details. Please try to move the 2.4G devices to the main 2.4G network, let me know if the issue persists.

@Huffmanfam 

Hi, are your devices that disconnect from the 2.4G network connecting to the guest network or main? As Prof mentioned, his devices are connecting to the 2.4G guest network, and he is testing with the main now.

How many devices are on the 2.4G network? Try to modify the wireless security on the router to WPA2-PSK, AES, run a Wi-Fi analyzer to review the channels around, then choose a clear channel (Smart Connect should be off first) on the router to monitor.

@Huffmanfam 

How many devices are on the 2.4G network? Try to modify the wireless security on the router to WPA2-PSK, AES, run a Wi-Fi analyzer to review the channels around, then choose a clear channel (Smart Connect should be off first) on the router to monitor.

@Prof 

Hello Prof.  @Kevin_Z  asked me to jump in and respond to your last post as well.

Regarding your question about guest network isolation.  As long as this option is not checked the device on the guest network will not see or access anything on the local network.  The option above allows them to interact with each other but that should be it.

Regarding the TrendMicro Powered HomeCare.  The security or Antivirus portion protects against Malicious content, based on TrendMicro's database of reconzied threats.  All content that passes through the router will first go through these services and anything that is flagged will be blocked.  If someone or somehting is trying to break into the system, typically the reason why IoT is recommended to be on a isolated guest network, the IPS portion in most cases be able to detect and block the intrusion.  Finally the quarantine feature.  While the devices access isn't restricted if a device is found to be comprimies it will be automatically quaritined from the network thus protecting your other devices from being infected.  Now nothing is fool proof and these actions are only possible based on TrendMicro's datebase, which is regualarlly and automatically updated. 

No other devices should be needed.  However one thing that has me a bit concerned is the stability concern you first mentioned when you updated recently.  While all software updates do go through rigoruos testing, real world application is sometime not possible.  If you would be intrested I can refer this to our system engineering team.  They may be able to do a remote session with you and do testing that could determine if there is a bug in the software that was perviously not known.  If you are interrested please send me a message with your contact infromation and a good day you are available.  Our SE team is based in China so they do work hours opposite of US normal working hours.  They are also techincally a day ahead of us so working days for them are our Sunday through Thrusday any time after 5pm Pacific.  Just give me a day and at least a 1 hour window you are available and i will get it scheduled.