@d_jemme 

When you state you connected tech support here can you elaborate more? Did you direct message someone?  If so who?  

Currently, the PX50 is only sold in a 3-pack. So to get additional units you would have to have to purchase a full set.

One option I have seen customers go within old house situations is running Ethernet in the attic or under the home in its crawl space.  I do not know how feasible that would be for you but if its an option then you could use any Deco and just have them all connected to a switch through Etherent.  That would also give you the best possible throughput as well.