I've moved your question to the NPM forum which is more suitable for this kind of inquiry.
I've done what you have before and it works. Vs a custom property this method puts the responsibility on you to add the subnets or IP's to the alert logic but anything added that matches will alert without needing any custom property changed.
With the new web-based alert console you can create complex conditions but I don't think it would be any better then what you have above.
Your problem is that the lines with IP address need to be indented so that they are under "Trigger when ANY of the following apply". Currently all your conditions are under "Trigger when ALL of the following apply".
Delete the lines with IP address on them and also delete the last line in your example. Then highlight the "Trigger when ANY of the following apply" line and click on Add and select Simple Condition. And then add in each one of your IP address lines.
So your trigger needs to end up looking like this.