Adding a RBS20 satellite to RBR40 Router + 2 RBS40 Sats

Hi,

I've been having problems with my Orbi system since reworking the back of my house (well more problems, it's never been great). The groundfloor is about 750sqft with two further floors of 500sq ft on top.

Now a RBR40 and a Couple of RBS40s used to cover everything fine, but with the remodelling I'm struggling to get a signal in the open plan room at the back and to the Nestcam that is on the back wall beyond that.  That wall is concrete block and brick, so pretty thick.  But (when its working) I get 50mb down and 15mb up on the internal side of the wall.

I've tried adding a new RBS20 satellite to boost coverage in that room, but it just seems to bring the rest of the system down whenever its plugged in. It updated to the latest latest firmware (v2.5.1.16) fine, so can't see why its causing problems.

I know Orbi systems are fragile and tend to take a while to settle down after making any change whatsoever.  But this is driving me mad, especially in lockdown, when everyone need a stable connection.  I struggle to understand why a system like an Orbi with 2x sat can't cope with a house that is not much more than 1500sqft, which the old stock modem/router my network gave me used to just about do.

So question is should I persist with RBS20 and hope things settledown, or stick it back in the box and send it back to Amazon?