The chmod +x method usually works to get the simulator running on MacOS. Here is a general description of how to use it.
(The description is not specific for Freerider, but it's basically the same technique for all apps). You can find other tutorials on Youtube, search for "catalina app can't be opened" for example.
There is also some general suggestions in the user manual.
And you can find some other tips in the other MacOS thread.
If you still can't get it working, I would suggest going to a dedicated MacOS forum and see if you can get some help on how to run unofficial apps. The issue is not directly related to Freerider, it is a security feature to stop people from running software that is not officially endorsed by Apple.