@Strygun 

Hi,

If your US version of the AX55 is broadcasting its 5 GHz wireless network on channel 149 or above, then the European RE200 cannot see it, because those channels are not supported in Europe.
Therefore, make sure the 5 GHz Wi-Fi of the AX55 is set to a channel lower than 149. (in the webGUI this option is under "Advanced" -> "Wireless" -> "Wireless Settings" -> "Channel")

What exactly happens when you perform the WPS procedure?
Do the designated LEDs for WPS on the AX55 and the RE200 just blink for two minutes and after the two minutes are up they stop blinking without any connection having been established between the two devices?

  @woozle thanks for the quick response. The 5Ghz channel was set to Auto. I've now changed it to 120 (DFS) - there was a note about radar detection but otherwise it seems to have changed successfully. Do you have any recommendation on what channel, below 149, is best to select?

Before the channel change, I tried resetting the RE200 and running WPS. Lights on both devices blinked for a while and the RE200 seemed to end up extending the AX55, but not as a onemesh network. Instead, the RE created extended networks with the same SSID as the AX55 but with _EXT appended at the end. So then I reset the RE200 again, and tried WPS again. Since then, the WPS process hasn't worked - lights on both devices just flash a while and then stop.

 @woozle and for context - this is what I see in the Tether app while logged in to the RE200. As if the OneMesh is set up properly. But if I look at the network map while logged into the AX55, it says there are no onemesh devices.

  @Strygun 

To rule out some corrupted configuration settings within the AX55 you could do the following test.

1) On the AX55 go to "Advanced" -> "System" -> "Backup & Restore" and perform a "Backup" of the current router settings.
2) Perform a full "FACTORY RESTORE" from the same menu.
3) After the AX55 has booted up try again to add the RE200 via the WPS button procedure.

In case it succeeded this time (i.e. the RE200 shows up correctly in the EasyMesh menu of the AX55), then I am afraid you would then need to manually configure all the other settings of your router again.

If after resetting the AX55 it still doesn't work, then go back to the "Advanced" -> "System" -> "Backup & Restore" menu and "Restore" the settings from the backup file you saved previously. Your router should then be in the same state like it was before this little test.

  @woozle thanks for the suggestion. I'll give it a shot, but I'm traveling for the next few weeks so I'll post here then with the results. Meanwhile - any suggestion on what channel I should assign the 5Ghz band to? I'm guessing I still need to set that after I do the factory reset, before trying WPS.

  @Strygun 

Yes, given that the US AX55 might automatically select a 5 GHz channel that the EU RE200 can't work with, it's better to set a specific channel manually.

Best practice for choosing a Wi-Fi channel would be to check what channels neighboring Wi-Fi routers are working on and then pick a range that is either still unused or, in case there are no unused channels, pick a channel of a range that is the least crowded. (you probably know that the router isn't just using the one channel that you select, but instead a whole bunch of adjacent channels, depending on the selected "Channel Width")

There should be free software available for all popular platforms to do this. Just look for something called Wi-Fi Scanner or Wi-Fi Analyzer.

  @woozle Turns out, doing a factory reset made it all work. The extender was able to connect to the router using WPS (finally), and the HomePod Mini in the kitchen now has a stable internet connection. Thanks for helping me troubleshoot all this. Too bad it required the nuclear option, but whatever that did ended up making it all work like it was supposed to from the start. Thanks!