To connect to a Legacy base and use the MTH WiFi app, the TIU must be plugged into a LCS SER2 module.
From Lionel you will need the following in addition to your Legacy Base...
6-81326 LCS Serial Converter 2 (SER2)
6-81499 LCS DB-9 Cable with Power Supply
Please note the Lionel WiFi 6-81325 is NOT required to use the DCS WiFi.
Connect it as follows from the DCS Companion...