@maf1250 

BTW, I have added keywords and domain names in both the Whitelist and Blacklist fields. Is it possible that such contradiction may cause the rebooting?

Also, sometimes the devices affected leave suddenly and rejoin after a short period of time. 

UPDATE:

I have deleted the whitelisted keywords and domain names and tried to access a blacklisted domain on one of the affected devices. Instead of just denying access, the router rebooted itself!

This happens every time an affected device tries to access a blacklisted keyword/domain. 
Also, I noticed that the affected devices can't access other websites unless they are added to the Whitelist field, which is weird because it is stated in the information box that

"A. If you want the controlled devices to have access to all websites except certain ones, select Blacklist.
B. If you want the controlled devices to only have access to certain websites, select Whitelist."

@TP-Link 

Thanks for your swift reply. 
 

1. Well, I have two children, one has an iPhone and a PS4, the other has an android tablet, so it isn't an OS-related issue, I believe.

2. Yes, everything becomes normal once the bulb icon for the designated device is clicked.

3. Unfortunately, yes!

I will add the config file shortly.