@PaulB1 

Hi,

In the EasyMesh menu of the web admin screen of the RE900XD make sure that EasyMesh is switched ON.

Also, from the extender's web admin screen please click on "Quick Setup". The RE900XD should start scanning and after a few seconds show a list of nearby Wi-Fi networks. Does the SSID of your Optus Ultra 5G have a [Mesh] or [EasyMesh] label attached to it? 
 

By the way, does the Optus Ultra 5G have some sort of EasyMesh settings menu within its own web portal?

  @woozle 

I've tried what you suggested. But after i go through the quick setup, it setup up EXT SSID's and disables the "EasyMesh" flag.  This is what is does when i run through Quick Setup:

Then it disables the EasyMesh flag:

  @PaulB1 

Ok, it's good to hear that the router's Wi-Fi gets flagged [EasyMesh] after the scan, but it's bad that the RE900XD adds the _EXT in the following step.

From the screenshot you posted it appears the two SSIDs of the Optus might be identical. (i.e. "OPTUS_          6Z")

In case you have activated an option on the Optus router that causes the two Wi-Fi bands to be "combined", then deactivate that option so that the 2.4 GHz Wi-Fi and 5 GHz Wi-Fi have different SSIDs and then try again.

Other users have previously experienced problems connecting a TP-Link EasyMesh range extender to non-TP-Link-retail routers if the Wi-Fi bands were combined.

  @woozle 

Thanks for the tip. I have now try splitting the bands on the Optus modem so that the 2.4G and 5G are split over separate SSID's, but i am still experiencing the same issue. This is the screen i see when i do quick setup:

I found this link: https://www.tp-link.com/au/support/faq/3582/ which shows a similar screen, but next to the SSID it has "EasyMesh", which my screenshot does not show. My modem is supposed to be EasyMesh compatible, but maybe it's a problem with the routers EasyMesh support instead of the extender :( 

  @PaulB1 

Oh, I misunderstood you earlier. Sorry. 
I assumed you did see [EasyMesh] next to the SSID. 

If there is no [EasyMesh] label shown next to the SSID in the network list, then of course it means the RE900XD doesn't recognize the Optus Ultra as supporting EasyMesh and hence will not establish an EasyMesh network.

Can you please check a few things on the Optus?

Is the model number of your Optus Ultra modem "QN2891"? (usually printed on the underside of the router)

A picture I saw on the Internet showed a "MESH" menu in the web portal of the Optus Ultra. (in the "My Wifi" menu the fifth tab from the left)
Does your Optus Ultra have this menu? If yes, what can you do within that MESH menu?

If it doesn't have that MESH menu, then can you try to find any date stamp within the web portal of the Optus Ultra that indicates the release date of its currently installed firmware? 

The certification of this Optus Ultra modem happened quite recently (Feb 2024). So there might have been older firmware versions that didn't include EasyMesh support.
https://www.wi-fi.org/product-finder-results?keywords=Optus%20Ultra%20WiFi%205G

  @woozle 

It does have the "MESH" tab in the wifi menu, but there isn't anything you can configure or do in the tab:

The only hardware ID i can find is "NH8091HW1.0", which seems to correspond to the underlying manufacturer LTE's certification on that portal you sent https://www.wi-fi.org/product-finder-results?keywords=NH8091 . It does seem to say that it is EasyMesh compatible. The firmware version is "ZNH8091V1.8". It does not have any option to check for latest firmware and i've been constacting Optus numerous times to try and see if there are any firmware updates but they just say that it automatically installs new firmware when you reset the modem. Usually there would be an option to check online for any updates and install them, but this modem only has an option to upload a new firmware from  file upload.

  @PaulB1 

I am sorry, but I have no more ideas what else could be tried.

According to the Wi-Fi Alliance's product finder ZTE has already certified far more than a hundred "EasyMesh" devices, so we should assume they know their stuff.

And as far as EasyMesh is concerned, devices that support EasyMesh should be able to pair with each other by performing these "easy" steps:
1) reset the device that is to be added as a new secondary/slave/satellite node to factory default settings
2) bring both devices fairly close together (troubleshooting guides usually mention within 2 meters)
3) start WPS on the main device and then start WPS on the device that is to be added as secondary/slave/satellite node (although in theory the order shouldn't matter)