Router blocking some websites

[MINIMD]

Router: RAX10 (latest firmware) 2 years have been used. It started happening two weeks ago. 

Technical level: Intermediate or advanced in troubleshooting

The router is connected to 6 Windows/Macs via Wifi and ethernet. 

- A few websites are getting 403 Error or Error time out (3 or 4 sites all legit site)

When Connected directly to the Cox Modem, those sites have no issues. The router has all default settings and is reset. But when I connected to my iPhone via wifi (diable cellular data), I could access those sites but not from desktop machines (all 6 of them).

According to a previous post, cloning a Mac address will fix the issue or replace the router. 

I'm interested in more about why it happened in the first place.

[FURRYe38]

What Firmware version is currently loaded?
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. CAT6 is recommended.

Has a power off for 1 minute then back ON with the ISP modem and router been performed since last update?
Be sure to restart your network in this sequence:
Turn off and unplug modem.
Turn off router and computers.
Plug in and turn on modem. Wait 2 minutes for it to connect.
Turn on the router and wait 2 minutes for it to connect.
Turn on computers and rest of network.

Has a factory reset and setup from scratch been performed since last FW update? A complete pull of the power adapters for a period of time after the factory reset then walk thru the setup wizard and setup from scratch. Recommend setting the default DHCP IP address pool range to the following after applying and a factory reset: 192.168.#.100 to 192.168.#.200.

Do you have IPv6 enabled on the router? 

[MINIMD]

Router: Nighthawk AX1800 RAX10 (version 1.0.10.110)

Modem: Arris CM8200A 

I did these steps: 

""""Has a power off for 1 minute then back ON with the ISP modem and router been performed since last update?
Be sure to restart your network in this sequence:
Turn off and unplug modem.
Turn off router and computers.
Plug in and turn on modem. Wait 2 minutes for it to connect.
Turn on the router and wait 2 minutes for it to connect.
Turn on computers and rest of network. """"

IP 6 is diable in Router.