RBK20 External addition

Hi Guys,

just a quick question if I may, I cannot seem to locate a compatible addon for my Orbi RBK20 system that can be located outside ? 

I live in the UK and have a daisy-chain setup comprising of the main unit that is plugged directly into my broadband router then two additional wireless satalites, while the reception in the garden is okay, I would like to strengthen this by adding another Orbi unit that can be integrated into my existing setup. I have external power so a plug-in would be okay, I just need to ensure that I am purchasing the correct compatible item. 

Many thanks in advance of any assistance you might be able to offer. :)