Hi Steve:
After reading your post I tested my MTH - Wi-Fi. I am using an iPhone 11 Pro with iOS 14.5 with 22 MTH Locos. I ran three locos and all is working. With that being said, I feel the MTH Handheld Controller is more reliable.
Yesterday I updated my iMac 27 inch to Mac OS Big Sur 11.3 / All is well.
In this photo I am running the C&O 492 Yellow Belly. The loco is now parked.
At this point the loco is doing 5 mph on the main line.
Hope this helps. Gary. 🚂