Now we know that we are experiencing issues purely with the router, we will now need to factory reset.
Please be aware that factory resetting will remove any exisiting settings you may have changed, such as WiFi names, passwords, port forwarding etc.
Find the reset button on the back of the router, it usually is a pin hole that you will need to hold down for 5-10 seconds. Once held, the router will reboot itself.
Has this resolved your issues?