Hello,
There should be no issues to ping 8.8.8.8, or any public IP/domain in your current configuration, as long as Fritzbox has internet access.
Could attach a troubleshoot file to your question to review device settings? Please, replicate the issue, then access router's WebUI, go to System -> Administration -> Troubleshoot section and download troubleshoot file from there. The logs in the file might provide more insight into the issue.
Attached files are private and visible only to Teltonika Moderators.
Best regards,