I was installing webthings with 3 bulbs configuration for one of my customers.
Everything went smooth until I was trying to conect external gateway address:

I was getting the error like this SSL_ERROR_ACCESS_DENIED_ALERT

When I reconnect webthings gateway to my mobile network, everything works fine for this url:
When I reconnect to my customer network again. I’ve get this error again SSL_ERROR_ACCESS_DENIED_ALERT.

Do you have any idea what was wrong or how to debug this issue?

That error generally occurs when your gateway is unable to connect the tunnel to our cloud service. You need to make sure that your gateway can reach the internet (e.g. ping, and that it can reach our server at, e.g. curl -o /dev/null -s -w '%{http_code}' should yield a 404.

Also, one more question. When you first set up your gateway, did you let it finish completely, to that point that it automatically redirected you to your new domain?
The next thing to check is for PageKite related lines in the log file, e.g. grep -i pagekite ~/.mozilla-iot/log/run-app.log.