Hello,
Please check your project Sender ID and API_KEY, they must match or else you will get that MismatchSenderId error. Kindly logged into your Google Developers Console-> Project -> APIS & Auth -> Credentials -> Public API Access -> Key for server applications ->
Then add that key into App42 Management Console -> Push Notifications -> Settings -> Android Settings -> Push Keys
After following the above steps, reinstall your App with all the new settings and run, it will resolve your issue.
Please check and let me know if you still have a problem.
Thanks,
Himanshu Sharma