I have the BT Whole Home 4 disc system in my home with the prime use of getting wifi into my office cabin.
My system topology is set to daisy chain the connection from the main disc to the one located in my office cabin. I spent time to get the optimal position between each disc so that they have all good signal etc.
On the odd occasion the disc in the cabin loses sync or disconnects from the network and goes red, that saying sometimes it stays blue but is not shown in the network map. The only way to resolve the issue is either a whole home reset or by switching off and on the final two discs in the chain i.e. the office disc and the next one closest to it.