This can happen to all manner of routers, including the extremely expensive carrier grade routers that ISPs use, the difference being the more expensive enterprise grade routers, and up, tend to come with features that either mitigate the damage (ie failover pairs with resilient connections, but that costs money, and requires two routers, and two or more connections) or reduces the likely hood of a crash in the first place (ie system resource monitoring, and low memory alerting). The out of band management features (that someone brought up elsewhere in this thread) also help in recovering from a failure, but out of band management requires a management connection (a separate data connection not reliant on the hardware being managed).
Someone else in this thread mentioned Draytek, and I have one of their routers, and am very happy with it, however, I very much doubt it would have prevented your issue. It sounds to me, like you have exhausted the running memory of the router, and caused it to crash. The RTSP connection you had to your cameras is a control connection, and the video data would have been sent over another protocol, I suspect something UDP based (but I could well be wrong) and UDP connection tracking is harder for a router to perform than TCP connection tracking, and so will have used more memory than the router would otherwise have used.
It is also worth considering the possibility your router has been hacked. I suspect this is less likely, but it is a possibility with remote management enabled (something that is always a risky proposition) especially if you are using an insecure protocol to access the remote management (http instead of the encrypted https) or you have used a relatively weak admin password. Using the default admin password and enabling remote management is asking for trouble. Consumer grade routers do not tend to come with much in the way of access accounting or tools to prevent brute force attacks.