Step1: Disable the antivirus and firewall check the internet connection.
Step2: Go to OES do Admin login
Step3: Do the Data Synchronization by selecting the course.
Step4: Log out the admin login and do the learner login your issue will get resolved.
Note: If issue persists, please generate the SOLAR ticket with following details.
Learner Code-(those are facing issue)
Screen shot of the issue-
Updated mobile number.