Normally when I’m having issues with a router the first thing I’ll do is perform a hard reset of the router. This can be accomplished by making sure the router is connect to power and is on then inserting the end of a paper clip into the reset hole and holding the reset button down for 20-30 seconds. Once the router has been reset, you will have to reconfigure it. Make sure that during the reconfiguration process you update it’s firmware to the latest version.
Hope this helps!