ATC doesnt work in A350

[JoeGor]

Although I have connected to VHF, the default ATC in MSFS still doesn’t work! Help! At the beginning, before take off, the ATC was performed well. However, after take off, it keeps telling me to reset transponder since my transponder appears inoperative or malfunctioning!!! Even though I have reset the squawk as required, it still doesn’t work! After a while, the ATC told me that IFR was cancelled and transitioning to Charlie airspace!!! And then, I couldn’t press anything in the ATC box and there is no options provided in the ATC box as well!! 

[jaybird]

Of course, given the quality of ATC one might argue this is a feature 🤣

The transponder is controlled by the MCDU, it's another of those gotchas, top bar see rather than FMS look for SURV, there you will find what you need.