We just had just problem while setting up Ops Manager at a client whose policy is to have SELinux enabled on all production environment, so disabling is not an option.
We needed to have dbPath set to /db and so SELinux was blocking even though permissions were correct.
The docs at https://docs.mongodb.com/manual/tutorial/install-mongodb-enterprise-on-red-hat/#install-enterprise-rhel-configure-selinux don't reflect the required solution:
sudo chcon -Rv --type=mongod_var_lib_t /<dbPath>
Maybe it could be added and explained under the section on semanage port