I (presently) have router between my modem and my Airport Extreme.
In a conversation with an Apple wifi tech (at Apple Care) yesterday, he advised that if one has a router in the link, as I do, then an Express will work ok, but it will not extend the range of the Extreme.
Is that so? Or, might one of us (or both) be confused?
Harv 27" i7 iMac (10.13.6), iPhone Xs Max (12.1)
Those who can make you believe absurdities can make you commit atrocities. ~Voltaire
Be careful about language here. I suspect you and the Apple tech were having two different conversations and neither of you knew it. (That happens between my wife and I all the time.) There is a difference between the range of a device such as the Airport Extreme or Airport Express and the coverage of the Airport (WiFi) network. While an Airport Express can not extend the range of an Airport Extreme － the range of the Airport Extreme is what it is and no more－ it can extend the network coverage by setting it up at a point near the edge of the coverage provided by the Airport Extreme that is originating the network and configuring it to Extend a wireless network.
My house is a long Texas ranch style and the office, where the Time Capsule (a.k.a. Airport Extreme) is located, is at the far west end. The network it originates does not cover the far East end of the house where the master bedroom is located. So, I have an Airport Express configured to Extend a wireless network plugged into a wall socket in a room near the middle of the house but still within the coverage of the Time Capsule. The Airport Express in turn provides reliable network coverage to the East end of the house. When I have my laptop or iPhone in the bedroom they connect to the Airport Express and not the Time Capsule and when I am near the office they connect to the Time Capsule. For all practical purposes it is the same network and I cannot tell any difference without launching AirRadar to see which device I am connected through. Interestingly enough the EyeTV, which is near the middle, sometimes connects to the Time Capsule and other times to the Airport Express. It makes no difference in performance however.
Edited by joemikeb (10/14/1009:01 AM) Edit Reason: spelling correction