Can you please also share a screenshot of the ATK Error history window (View → Error history)?
I only looked at some tangotest attributes 
Thank you for the screenshot.
It looks like you are having 2 different problems.
You are encountering the famous heartbeat problem. So your client (ATKPanel) is trying to reconnect to your device server every 10 seconds and is doing a synchronous read_attribute call every 10 seconds.
So you should see an update of your attribute every ~10 seconds on ATKPanel.
Then, you have some other errors “Read value for attribute xxx has not been updated”.
This can happen if the attribute value was not set in the device server attribute read method.
(attr.set_value() was not called in this method).
Could you please check that the attribute value is correctly set in the device server read methods for the attributes reporting this error?