The older Centralite/SmartThings devices are very picky on the Zigbee channels. When I moved from ST to HE in Feb/March 2018 I struggled to pair my ST leak sensors and motion sensor when using channel 17. Pairing for each device took forever as mentioned in this post:
This could be why you are struggling with channel 18. 20 has worked well for me, but my Zigbee devices are still on a C7. Just wanted to make you aware of the channel selection for these older devices.
