Help - about to dropkick my Deco system into orbit

I'm tearing my hair out here, hoping someone can help with this godforsaken mess that is the Deco app. 

I have Deco set up for Mesh - 3 X55s as the main network, and subsequently added some M4s for a bit more coverage. It all worked fine, until I stupidly reset it all when trying to work out why my network wasn't working properly (it was the ISP modem). 

The X55s are all up and working fine again. I have also managed to get 1 M4 attached. I cannot, for love nor money, get the other M4s to connect - the furthest I get is "we've found your device" (which happens about 1 in 10 times I try), then "applying settings", before it tells me "failed to add this device". 

I'm hardwiring them all together; the system's in AP mode; and in the router I had static IP addresses assigned to each, but have removed those in an effort to try to get it to play. I've also, following advice on reddit, tried to set up one of the M4s as its own network and check its f/w is up to date, then reset it and add it to the existing network - no dice. 

So what is it I need to do?! Which god do I have to pray to to make this effing thing just. effing. work?!!