@Tuvestam you can only add the appropriate devices to each network and a combined network is simply a collection of functionally specific networks. i.e. MRs in a wireless network, MSs in a switch network etc. I'd have thought you'd need a network type of sensor, but are you saying that it wants you to add it to an appliance (MX) network?
One other thing to check is that the network that you are trying to add the sensors to has the MRs on 27.5.11, not 27.5.1 as you need the .11 version. It should be offered as an upgrade once you have claimed the MT