Fibrenest Broadband Child node problems

I have fibrenest FTTP service . The router supplied is a linksys wh03 v2 and child nodes can be purchased from fibrenest at around £100 each.
I need 3 nodes for big house,
Argos sell a 3 node pack of linksys wh03 v2 fo £200. Ive purchased those but cannot get them to link to the Parent Fibrenest Node.
Is there a software block on the Fibrenest router to prevent 3rd party routers being used.
Fibrenest state in the service conditions that they dont prevent other equipment being used on their network.
Any help would be gratefullly recieved

How are you adding the extra child nodes - are you logged into the base Velop or are you using the app?