For those who are unable to make it work with custom DTH on SmartThings v1 hub, it is possible that the issue is caused by the radio signal interference between zigbee and your wi-fi. Try out the following - as it was the only thing that resolved my issue after multiple days of debugging.
Refer to this post to figure out which Wi-FI channel may be causing interference with your zigbee signals - FAQ: Networking and Reducing Channel interference between WiFi and Zigbee. The v1 hub operates at fixed zigbee channel and you can see the channel by viewing the status of your hub once logged into the dev account. For v1 hubs, these zigbee channels can’t be changed, so best option is to change your wi-fi channel. For me, zigbee channel was 20 and wi-fi was 11. So had to change wi-fi to channel 1 and immediately the problem was resolved.