This should, probably, be in the DCS forum. Having said that, from your description, the problem is that the Wi-Fi is not finding the TIU. It is very likely that there is nothing wrong with the locomotive(s).
Check the connection between the TIU and the Wi-Fi.
Is your TIU powered through Fixed Channel 1 or an external power supply? The fixed channel can be problematic since any disruption of power will disconnect it from the Wi-Fi.