@Daregt Please reset the 850RE first; how to reset: With the extender powered on, use a pin to press the RESET button for 1 second. The extender will reboot. Then please watch the setup video or...

@easyfriend4 Thanks for your updates, glad to hear that you figured it out. Good day.

@devanirnf Please provide more specific symptoms of the connection problems, like the unstable connection or slow speed. What is the led status of the Archer C9 when the connection problem happened?...

@Stevorama Hello, sorry for the trouble caused here. You can tell us the current problem and then we can do some troubleshooting and try to fix it. Best regards.

@Panditji_N450 Please login to the web UI of the TL-WR940N and go to status page to verify whether it has the valid internet IP address. And please check the operation mode of it, is it wireless...

@Simon1234 You can refer to page 43-44 of the user manual to configure it as the 3/4G router mode. https://www.tp-link.com/support/download/archer-vr900/ With the USB modem plugged into the USB port...