I tried both methods and all of them are working fine
1) for ASTRA theme, the “APM: True” flag that unfortunately requires the need to create a child theme. On this method I have been struggling with functions.php code. I’m not too familiar with PHP so I just copied a sample file and use it, but I’m not sure about final results.
2) with snipped code suggested by Weston, ,it’s simpler but again I had to look where to insert it. At the end I used this plugin https://it.wordpress.org/plugins/code-snippets/, then it works perfectly.
3) then I installed Neve theme and I found that is already configured in the right way, so it shows up automatically after installation in the reading themes list.
Finally, I noticed that both themes configuration is limited as, when you configure AMP options, many of them are locked because their dependencies from main configuration, so you can change it only within desktop/normal panel. Hopefully, theme developers will find some way to make it better.
Anyhow many thanks to all for your help.