@DianeL 

Hi,

When replacing the main Wi-Fi router inside ones home, then in most cases it should be sufficient to set the Wi-Fi network names and Wi-Fi passwords of the new router to be identical to the ones of the old router that is to be replaced. That should allow most client devices, including the RE200, to connect to the new router without requiring any further action.

However, if the above doesn't work, then the preferred way to re-setup a RE200 would be to reset it to factory settings by pressing and holding (about 5 seconds) its small "Reset" button and then use the official TP-Link user guide to set it up from scratch again, i.e. the same procedure that was used to setup the RE200 when it was brand new.