"Request participants" custom field is not available when configuring a security level. So there is no way to both enable issue security and allow request participants to view these issues.

Documentation warns about this but offers no workaround:

Note that enabling these settings does not bypass JIRA issue-level security. If issue-level security (e.g. restricting an issue to only be viewable by the reporter) has been applied, participants may not be able to access their requests.

There is an available workaround using Automation for Jira, documented in this KB:  Setup Issue security level to work with the Request participants on JSM issues using Automation

The workaround is to make use of Service Project Customer - Portal Access security ** group to meet this requirement:-