Can I use two Airport Express routers to extend an Airport Extreme network?
I recently was able to do this successfully, adding a 2nd Airport Express with a wifi-only connection to an existing network which had RS232 lines to a modern Airport Extreme and an Airport Express. I'm using the latest Mac OS. What I found most effective after plugging the new unit it was to restart the Mac (Mac Mini circa 2012) several times so that it would, eventually, recognize the new Express hardware. I did set the TCP/IP Configure IPv6 to link-local only but I do not know if that made any difference. Once everything was working I changed back to Configure iPv6 automatically.