Orbi RBW30- Can access device to update firmware

I have recently bought a second hand Orbi RBK30 (consisting of 1 RBR40 Router + 1 RBW30 Satellite)

The Router to Satellite connection keeps dropping out, even when they are only 2 metres away from each other.

I tried to carry out a firmware update & the Satellite is reported as being an RBS40 and the software version is just listed as -----

Each time I try to do the automatic software update it fails.

I have tried to carry out a manual firmware update, but the browser fails to access the device 

I am assuming that the previous owner of the RBW30 tried to load the firmware for the RBS40 and has possibly corrupted the firmware in the RBW30.

Because the RBW30 has no ethernet connections I can only communicate with it via WiFi.

When I look at the Connected devices it shows Good 5Gh link to the RBR40; and then it will show as disconnected on the device list (but it doesn't have the magenta light up on the RBW30 to report a disconnection).

I am running a Windows 10 laptop & I use the latest firefox & edge browsers.

The Orbi RBR40 Router is running Version 2.3.5.32 firmware.

My Internet connection is via a Plusnet Hub One | Software version 4.7.5.1.83.8.263 | Last updated 11/03/19.

The Orbi RBR40 Router is configured as an Access Point because I can't configure the Plusnet VDSL Router to just be a VDSL Modem.

Does anyone have any suggestions how I can load new Firmware, and which version I should start with (Do I need to go right back to the initial version & work through all of them, or is there an internediate version that is a full firmware load - the firmware files are.chk, not .img and it looks like they are patches, rather than full loads).