I'd like to state that for me, having a signal strength issue can cause weird things with the app. I wish it would state that there's a signal issue instead of saying "no TIU found" ! The worse thing about DCS for me are those generic messages that don't tell you the exact problem. You have to learn that they just tell you something is wrong, not where exactly.
You can test this on your system by getting a single engine on a track with good signal strength running well and then adding an offending piece that wrecks the signal. You may (should) start seeing those messages on the app pop up. I've even had issues caused by engines with dying batteries. Once you find the exact problem, seeing those messages will make better sense.
Using the app can be a much better experience. It adds, finds, and does other things faster than the remote. It is so much easier to name something for example. Also, You don't need to scroll thru an entire engine list using a clumsy thumbwheel to get the desired engine. I have a lot of engines so it's a big thing here. For us that got used to the remote first, switching to the app is tougher. Things must be done methodically so not to cause other issues. Like powering off the TIU before the controlling device. That can lead to it reverting to the house's router instead of the MTH network. That probably is different on each device?
I can't help but wonder why so many posters have issues and others don't. There was a very recent problem with the latest IOS version only that seemed to confuse the issues and the cure. It makes it tougher to diagnose the problem. So eliminating things has to be done properly and orderly to narrow down where the problem is exactly. I don't use IOS devices. I can't help a lot with them other than what I've seen on my Android versions.