new modem and router, existing EX6100 v2 and EX6150v2 won't connect

I was having spotty wi-fi, so replaced my Spectrum modem and my wifi (purchased a TP-Link Archer AX3000). They are both set up and I now have blazing fast wi-fi again. But I can't get my two range extenders back online. The 6100 uses the auto-detect system to prompt me to set up a new extender (I reset the extenders after they weren't detected initially) - I went through the steps and it said my wi-fi password was incorrect. Yeah, no it isn't. I double checked three times, PLUS I just set the password earlier today when I set up my new modem and router. I moved the extenders 4 feet away from the new router and still the same error. Then I tried to set up the newer 6150 extender with the Nighthawk app. Same deal. I see the lights blinking on the extenders as I work - all happy green lights - but no luck. I can see the new extended networks in system/wi-fi on my devices, but they say they haven't been configured. I'm using an iPad, a MacBook Pro, and my iPhone. I've restarted the modem, the router, the computer, the iPad, the iPhone.  Oh, and I've also tried "forgetting" the extended networks, then starting this process again. The range extenders are new-ish - one is 5 months old, the other is maybe 3 years old. Any suggestions for me? Thanks.