It’s unusual for a device to have a range issue that works for six hours and then stops working. Unless you reached a point where the batteries died and that doesn’t sound like that’s what’s going on.
More commonly that would be an interference problem. In particular boosted Wi-Fi can interfere with ZIgbee.
I Have a major problem at my house every afternoon about 15 minutes after the school bus arrives where some of my Zigbee devices drop off-line. I’m pretty sure one of my neighbors has boosted Wi-Fi, their teenager gets home from school, and the Wi-Fi traffic zooms up.
You can also get intermittent interference. It’s pretty much the same principle as when you turn on the microwave and Netflix starts buffering. As Soon as the Microwave goes off again, everything is fine. Even though the microwave is not actually on your Wi-Fi network, it’s causing interference.
If the problem is local interference, it could be very hard to diagnose without professional tools. Usually the easiest thing is just to put in a repeater nearby and see if that helps. I can understand why you don’t want to spend the additional money though.
I know that doesn’t help very much, but it might give you a place to start researching.
You can also write to support@smartthings.com and ask for their help in diagnosing the signal strength of the device it’s getting lost. They might be able to see something from their side.
I would also definitely go back and post in the original thread where you got the device type and see if the author or anyone else using it has any more advice. Sometimes are quirks specific to a particular device or device type that it helps to know about and asking in that thread is the fastest way to get the attention of people who are actually using it.