I am having this same problem with an out of range Xiaomi door sensor only paring to the coordinator (conbee2). I also have installed this sensor in a waterproof enclosure. I think the signal is struggling for this sensor.
I just tried paring the sensor on the other side of my house 1 inch from a hue bulb. The map shows it is still paired to the coordinator. I have to say I doubt it paired to the coordinator from all the way across the house and not the hue bulb. I am going to test this much further from the coordinator tomorrow.
I also have 6 of the IKEA repeaters.
After some time I am now seeing that the Xiaomi Aqara Door sensor is in fact connected to both the coordinator and a strong signal to the hue bulb!
Thanks for the reply. Have you noticed more reliable performance?
It is responsive within a second or so. It seems to like to be pared to the hue bulb. I should have invested money in 3 hue bulbs instead of 6 IKEA Range Extenders.
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I have a lot of the Ikea range extenders and Xiaomi (Aqara) sensors of all types, no problems at all. But yes, the Aqara sensors are not very good to connect to the best router by them selves.
Is there a way to “force” an Aqara sensor to link to a Tradfri signal repeater? I’ve got two in the garage and although they are paired with the Coordinator (CC2531) the link strength is zero and they rarely update. I’ve installed and paired a signal repeater between the two, but it seems to have done nothing, everything is still connecting directly.
Re-pair the aqara sensor close to the Tradfri signal repeater.
Do I need to delete them from HA first or anything, or just hold down the button on the sensor until the light flashes?
Just hold down the button. No need to delete in either HA or zigbee2mqtt.
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Jepp close to repeater as francisp says is important, Aqara devices has a tend to try to connect directly to coordinator even if it is far away compared to a more nearby repeater.
But i think maybe deconz is lying sometimes when it says that Aqara device goes directly to coordinator… Trådfri bulbs is often showed as repeater for Aqara, but not always the trådfri repeater. Does not make sense, but i think it is like that.
I have both cc2531 (Zigbee2mqtt) and Conbee II, i use the Conbee II for the Aqara devices, since i started out with Conbee II. CC2531 was added later.
FYI, most advice on Zigbee range extending can be found here -> Advice on Zigbee range extending
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Does anyone have advice on nice replacement for Aqara sensors ? especially thermometer and door sensor ?
I have many of these sensors, at least 10 of each. They are cheap, tiny, reliable, with a very long battery life. Almost perfect.
However, when uses with Zigbee2mqtt they do not change of parent node, even if the quality is really bad or the parent node dead. That’s a problem.
I’ve had success with Konke devices for thermoter and PIR sensor. Juste marginally more expensive and bigger (but not much), reliable also. And they seem more ‘Zigbee protocol’ compliant as they do change of parent node when they need.
But I didn’t try any door contact sensors.
Whar are your choices ?
I have 2 Blitzwolf Zigbee BW-IS4 Temperature and Humidity Sensors, little more expensive then the Aqara, but they have a nice display.
They’re looking good indeed ! 
Did not know that there were thermometer with display compatible with Zigbee2MQTT.
And do they switch of parent node when needed ?
Yes they do.
Thanks.
Will buy one to try.
Anyone founded a way to connect automatically Aqara sensor to best zigbee coordinator?
Having this issue now. For some reason the Aqara motion sensor placed all the way in the garage is losing range, and not detecting motion. Purchased an innr smart plug hoping to use it as a repeater, but having trouble getting it to sync up with the motion sensor. The door sensor however has synced up to it just fine. Anyone have any other suggestions in improving zigbee range?
First see PR for general tips → https://github.com/home-assistant/home-assistant.io/pull/18864 (latest revision at → https://github.com/home-assistant/home-assistant.io/commit/970295a277e8f01d3ee39eeeaacf453625b988d3 )
also of couse read and follow → https://www.home-assistant.io/integrations/zha#best-practices-to-avoid-pairingconnection-difficulties
Suggest buying one “IKEA Trådfri Signal Repeater” for each area to build a good baseline for the mesh:
https://www.google.com/search?q=IKEA+Tradfri+Signal+Repeater
Alternatively buy a few of ITead’s “Sonoff Zigbee 3.0 USB Dongle Plus” and flash them with Zigbee Router firmware → https://community.home-assistant.io/t/iteads-sonoff-zigbee-3-0-usb-dongle-plus-based-on-texas-instruments-cc2652p-20dbm-radio-mcu-now-sold-for-14-99/340705
Known limitation with Aqara/Xiaomi firmware. Only way is to manually re-pair device at final location.