Orbi RBS 750 Constantly disconnecting

I've been experiencing disconnects with one of our Orbi satellites for months. We thought it was our internet service but have established that devices connecting to the router are fine.  

Our set up is as follows:

Orbi RBR750 connected to Virgin Hub 3

1 x Orbi RBS750 (1) connected via wireless to the router. Located in the house, about 35 feet from the router

1 x Orbi RBS750 (2) in a garden room located about 30-40 feet from the house. This is connected by Cat 6 to the router.

All 750 products are on firmware V4.6.5.14.

The disconnects only affect the satellite in the garden room. Any devices connected in the house - to the router or satellite (1) are fine. The disconnects are random and we can go for a couple of days without one then we will get disconnects every 120 seconds to every 5 minutes or more. 

When they occur, they can be for a few seconds or a few minutes. The satellite seems to be the link as all devices in the garden room will disconnect (TV, any iPads and, crucially, the PC). 

We have tried:

• Using a second cat 6 (the garden room has two from it to the router position) to connect the router to the garden room satellite
• Using a new (loose, ie, the other cables are fixed in place) Cat 6 cable to connect as above
• Changing the satellites around, ie moving 1 into the room, then changing back to 2 when this didn't work
• Factory resets and start from scratch.  At least 3 times
• Connecting the garden room (2) wirelessly. This can be achieved with the doors open to the garden room and house and the signal varies between good and fair

None of the above work and having spent over £600 on this system, my patience is limited, not to mention my business is now starting to suffer (I now do a lot of work from home). 

Any suggestions would be appreciated. 

Tim