This wont stay connected. Aqara RTCGQ11LM. I added this using information from this forum. Anyone know what to do to make this stay connected?
I believe @JDRoberts has said this is a common occurrence with the Aqara motion sensors on ST. It’s not that the device is really offline, only that ST hasn’t received an update from the sensor in the expected period of time. His suggestion has been to use an Aqara bridge to bring the devices into ST to alleviate the issue.
My personal experience has been that aqara batterypowered Zigbee sensors brought into SmartThings via matter over bridge don’t have the reliability issues that I had when connecting them directly. It’s a local connection And no custom code required. 
I used to have a load of these until I binned them. Like many Aqara sensors of that period they had a relaxed approach to the standards which could cause problems. In particular they were quite fussy about repeaters. They got on well with the Ikea Tradfri so if you could pair them in situ with a Tradfri nearby they were a bit better behaved. This is how I ended up with a load of Tradfri outlets which I am now trying to replace before any more fail. Other repeaters also worked, just not any repeater.
Aqara and SmartThings seem to be new best friends as far as the more recent products are concerned, but I still think the best idea is to use them with an Aqara Matter bridge as @JDRoberts suggests.
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My experience is that Aqara/Xiaomi sensors lose connection due to the battery losing power, despite the app reporting no loss. Change the battery and reconnecting brings them back. Try changing the battery.
Could be the battery in some cases, but I don’t think that’s the usual reason since I can have an identical model with an identical model battery working through the aqara bridge and it has no problem staying connected.
Also, I always use a battery tester before changing batteries and the voltage level has been fine on the ones that SmartThings was marking as off-line.
I think the more likely reason is one which has been discussed on other platforms as well: the Zigbee specification said that devices were supposed to report once every 60 minutes and in the early generations, aqara set up their devices to report in the 60th minute. So before the 61st, but after the time when some platforms, including SmartThings, would mark them as nonresponsive. 
I frequently had these issues with the Aqara & Sonoff sensor going offline, after adding the smartthing station hub not facing this issue.