Help - Massive loss of speed from Orbi Satellites using Wi-fi

Hi,

I have an RBS850 router with 3 RBS850 satellites and 3 RBS750 satellites. Our house is 4,500ft²+ with large garden which requires wi-fi coverage for CCTV etc. The house is 350 years old so has very thick solid stone internal and external walls (1m thick in places) so wifi degradation though walls is huge, hence the number of satellites.

The whole system was working perfectly until about 10 days ago when the satellites which are connected by wi-fi have suddenly reduced in wifi speed. If I'm connected to a satellite which is hardwired using Cat 6e cable I get +350mbps, when connected to a satellite which is only connected by wifi back to the router I'm suddenly lucky if I get 6mpbs, 10 days ago I used to get +250

mbps. Absolutely nothing has changed in the house, no new devices, no new changes I just noticed a SKYQ mini box which was connected by data cable to a satellite using wifi suddenly stopped working, it can't stream the content.

I've tried switching satellites round, moving sky boxes, turning off some of them. Nothing works. I'm about ready to throw the whole Orbi system in the bin.

My previous Orbi system in my old house did something similar after a short period, just stopped working, but I've had this one less than 6 months.

Everything ap[pears to be using v4.6.3.16_2.0.51 firmware.

I'm hoping someone can help I'm at my wits end! 

Thanks