When I have all devices dropping off the internet this is a major problem

What devices are dropping off the internet? Wireless and wired devices? 

What router model is this? 

What FW version is loaded on the router? 

What is the Mfr and model# of the Internet Service Providers modem/ONT the NG router is connected too? 
Be sure your using a good quality LAN cable between the modem and router. CAT6A STP is recommended. 

Be sure to disable any MAC Address randomizers on phones and pads while at home:
NETGEAR Mobile Applications and Android/Apple/Windows Devices FAQ | NETGEAR Communities

Is IPv6 or Protection Engine enabled on the router?

Has a factory reset and setup from scratch been performed since last FW update?

Details are key. Just because ALL devices drop off, doesn't mean its a router issue. We dont even know what router or modem its connected to. And when all drop off, we also have to check the signal from the ISP as that's suspicios until proven otherwise. So details of your setup, what actually happens, what you've tested, and if the ISP has checked it (and how)

Here is the latest snippet and it is consistent in the log.

[admin login] from source 192.168.1.134, Saturday, November 22, 2025 17:38:27
[DHCP IP: 192.168.1.213] to MAC address ee:63:1d:c6:59:ef, Saturday, November 22, 2025 17:37:43
[DHCP IP: 192.168.1.218] to MAC address 00:19:ba:1a:04:4d, Saturday, November 22, 2025 16:59:50
[DHCP IP: 192.168.1.218] to MAC address 00:19:ba:1a:04:4d, Saturday, November 22, 2025 16:59:48
[Internet connected] IP address: 100.97.40.232, Saturday, November 22, 2025 16:54:39
[DHCP IP: 192.168.1.213] to MAC address ee:63:1d:c6:59:ef, Saturday, November 22, 2025 16:54:05
[Internet disconnected] Saturday, November 22, 2025 16:53:33

1. Wired and wireless devices are dropping (laptop, TVs, security system, cameras)
2. Nighthawk BE18000 WiFi 7 Router Model: RS600
3. Downgraded to Firmware Version V1.0.5.12 with few disconnects, however still disconnects and reconnects
4. nbn NTD Model CM8200B P2
5. LAN cable already checked and replaced (no change)
6. Phones and iPad set to Fixed
7. IPv6 - Disabled
8. No a factory reset and setup not performed from scratch since FW update (not a good option)

When the log shows "Internet disconnected" at 16:53:33 and then "Internet connected" at 16:54:39 (6 seconds later) that points toward an issue with the ISP connection.

Coaxial cables often experience transient problems from physical damage (weather, rodents, etc.)

It might be useful to contact nbn about these log entries.

I have reset the router using the Erase option under Admin and set up the router again with this changes. It appears to be OK at the moment, with no disconnections over the past two days. Is there a way to limit [DoS Attack: RST Scan]?

No.  People (and robots) on the internet continuously scan public IP addresses. Page 80 of the RS600 user manual describes an option to stop logging the detection.

https://www.downloads.netgear.com/files/GDC/RS600/RS600_UM_EN.pdf

Even before Netgear introduced the Protection Engine (and whatever it does, or doesn't do), the router firewall simply ignored attempts to connect from the internet unless the user deliberately forwarded specific ports to devices on the LAN.  There have been posts on the Forum in regard to some of the RS routers "rejecting" connection attempts rather than ignoring them.  This is often described as ports being reported as "Closed" rather than "Stealth".