All I can say is I no longer have issues pairing them to Sonoff router for a year now.
Aaaaaand it is gone again…
I will try to return the device to Amazon as defective.
Strange observation from my side. I went the route of ‘resetting’ the sensor by removing batteries for whole night and readding sensor to deconz. Did not worked, I was not able to resurect it in add-on. BTW, at least in deconz, it is not necessary to remove and add again device. it is possible to go through add device process without prior removing it and this ensure all device/entities settings are retained. I tried several times to add it, bu success…
But then the same evening my LAN core switch went rouge, effectively messing communication between all networked devices, including ESXi server when HA VM is installed and Synology NAS, where the MariaDB holding all history is installed and also works as iSCSI datastore for all my VMs images. Not healthy situation, so I decided to go through the process of proper shutting down all devices and starting them in the right order, so that all connectivity can be established properly. After 20 minutes of disaster recovery I found everything working fine… including questionable Aqara sensor, that started to be visible automagically in deconz and HA! So this makes me thing that the problem was not with sensor, but with deconz add-on that required full restart (normal HA restart does not restart deconz, as it sith in docker). So far ~24 hours and sensor is still there. I’ll report progress.
EDIT: today morning I found sensor to gone away again… 
Same issues with aquara door sensor and leak detectors. I have a bunch and only have issues with 3 particular devices. They drop off every day or 2 and require a complete reset. I reset device by holding button and re pair from homeassistant and it picks it back up as normal. Didn’t have a single issue with Iris (centralite) devices these replaced so thinking something is wrong with aquara as a brand maybe. I’ll follow this thread for any advice or follow up.
Well, still different type of installation (deconz), but I can only report that after update to the latest version of deconz and using new DDF file specific to Aqara T1 it is now stable - testing for ~5 days with no single drop. More over, I added to my system 3 more T1s, that previously I was not able to integrate. With new deconz version inclusion took 10 seconds each and all works fine.
So… perhaps you should consider change of platform?
Short update: I’ve sent my sensor back to Amazon and bought a new one, this one is running without any issues since 1 month now. Amazon refunded the full price.