@mogimogi You can refer to this link : https://www.tp-link.com/en/support/faq/2814/ and have a try.

@Heisenberg0709 You can check the power status of ER605, does it power off when the WAN gets down? You can also check the upstream devices connected to the router's WAN port to see if any reboot...

@RichMcD ER605 V1 is so old that it hardly ever gets firmware updates anymore. Why not buy a brand new ER605 v2.x to use new functions? New devices will get more support than older ones.

@acolmenares84 You can list the hardware version and firmware version of your ER605 so they can assist you.

@tsmithf Have you ever tried using your phone to connect to your WiFi? If you can connect to the 2.4GHz SSID and keep connected for a long time without disconnecting, the 2.4GHz WiFi works properly....

@Tedd404 Thanks so much!!