Hello Swapnil,
I am assuming that you are running a cloud version of the OS platform server and you are an enterprise customer. By default your domain name will be xxx-dev.outsystemsenterprise.com
Now you have changed the hostname to dev.mydomain.com and none of the mobile application is working.
I would recommend the following steps.
1. Rename the host name back to xxx-dev.outsystemsenterprise.com
2. Service Center - > Factory - > Applciations -> Your Application -> Native Platforms - > Environment hostname : Change to dev.mydomain.com. Re generate the application.
3. Add a CNAME in the DNS setting of mydomain.com. i.e Hostname = dev.mydomain.com and Canonical name = xxx-dev.outsystemsenterprise.com
This way both old application on the store( ipa , api ) and the new one will work.
Once the application is approved by respective stores and downloaded by users rename the host name to dev.mydomain.com