I got it working today, extending both bands, by switching the 2.4ghz channel from 1 to 11, and changing the extender power from High to Intermediate.

I think it was an issue with a fringe area, where there was a wifi connection, but no network connectivity.  The above changes seem to help.

On my AX1500 AP, I have 2.4ghz at low power and 5ghz at high power, and on the RE215 extender, Intermediate (no way to set the bands individually).

I can mostly roam in the house and stay on 5ghz, except for the very fringe areas.  I've got a lot of devices that are 2.4ghz only, though (microcontrollers, cameras).

  @david3 I have the opposite as I tried to avoid IoTs and wire everything Ethernet here, so it is a little hard for me to help with lack of experience...

I suspect it would be a good idea/may help to match an AX extender with your AX access point?...

  @jzchen Time will tell, but I think it's sorted out now.  I was using a wired access point instead of the extender, but I think the ethernet cable must be damaged.  I've had several ethernet cables installed in the wall & attic "go bad."  So I'm using wifi more now.

I actually don't have any AX devices, the AX1500 was just the new router available at the time I got it.

After more investigation, it appears that Windows and iOS (and possibly other OS'es) use DHCP in slightly different ways and consumer grade extenders do not support all of them.

So I will be returning this one and buying another Ruckus, using it with wireless backhaul

  @unmesh Hope it works out for you.

I've been doing some more reading;  what I've found so far, suggests that DHCP issues with extenders could be caused by the AP the extender is connecting to (and not necessarily the router/DHCP server).  If possible, the AP should be configured as an AP-Bridge.

With the TP-Links there's not much configuration you can do, but maybe it's an option on the Ruckus.

Though that doesn't entirely explain why it would work with some clients, but not windows.