I tried both .m3u and .m3u8
Are you running Channels and ADBT as containers? Sometimes the default bridge network doesn’t allow containers to talk to each other.
Channels DVR is running natively on a Windows PC and ADBTuner is in a container on a Synology NAS. Would that still cause this issue?
On your Win PC, make sure your PC & Synology can see & communicate with each other.
In that case you should be fine. Since Windows is your Channels host, and I assume you were browsing the ADBT web interface, the Synology ADBT container is accessible to the network.
But to test, on your Windows Channel Host, open a browser and navigate to your M3U8 link:
If it's working, it should download a file. If it does download, ADBT of hosting correctly and Channels should be able to see the link. If it does not, there is more to troubleshoot, starting with the Windows firewall.