Once this was done I plugged in the Wi-Fi extender pods again and everything is working fine. When I re-registered the speakers to my Alexa account they all worked perfectly again, and I was able to add them to multi-room groups etc.

What worked for me was unplugging all the Fibe Pods so the main router was the only Wi-Fi source in the house on that network, then I reset the main router, and reset all 3 of my Echo speakers to factory settings (by holding down the “mic off” and “volume down” buttons for 20 seconds). I messed around with everything for a while before finally figuring it out. I suspect the issue is that one of the Echo speakers was connected to one of the Wi-Fi extenders instead of the main router. I’m in Canada and have Bell Fibe internet with Bell’s “Fibe Pod” Wi-Fi extenders. I had the same problem and came across this old thread before finding a solution. Not sure if that's the true reason, but otherwise my whole network is able to connect to each other. I assume it somehow remembers the router it first connects to and identifies the network with it. Interestingly, also after moving it back to the other room and being connected to the repeater, it still works with the multiroom group. Tada - it lets me add it to the multiroom group now. Now I moved my Echo (physically) close to that router and re-initialized the WiFi connection. I realized that the other 3 devices were connected to the same router when they were initialized, but the one which wasn't working was connected to a repeater. In my case, I had 3 devices working together in a multiroom setup, but one Echo just didn't want to connect. The thread is old, but still the first thing that pops up when searching for it, so I'll add what I found after an evening of debugging: It seems like Echo is saving not only the SSID, but also the MAC (or something similar) when first connecting to a WiFi.

Multi room music alexa