I thought I'd throw in my 2 cents.
Hmmm... THREE different routers ALL exhibit the exact same behavior, and Asus couldn't find anything wrong with one of them. What are the odds of the exact same hardware failure occurring on all 3 routers? I think astronomical. So it seems like it HAS to be something in your specific environment that is causing your psycho mode. I'm thinking that it sounds as if the router is getting overwhelmed by something, making it unable to perform other duties - including running AI protection, or even handling normal routing duties.
What would overwhelm the router? I think you said that your psycho mode takes a while to manifest itself... Any chance that corresponds to your son's computer usage? Is your son running VPN, custom DNS, hosting Torrents? It's possible that something he's doing conflicts with your router settings. You are using DNS filtering so I'm guessing you're attempting to limit some type of internet content; maybe he's trying to "work around it". You could simply power off his system for a while and re-enable everything in the router and see if the problem returns.
I read that you have some static network addresses assigned. Do you really mean that you've created static addresses in the endpoint devices? If you really did create some hardcoded addresses in your end devices, then your DHCP range in the router needs to be setup to avoid those addresses otherwise conflicts could occur.
Or did you really mean to say that you are using DHCP address reservations?
Either way, what is your use case for having specific IP addresses? Are you also port forwarding? What is this being used for?