Thank you for reaching out!
As for the client not connecting to the APOS - the logs can be found by connecting to the router using SSH and running a command logread -f . This command will show the live feed of logs and might show the errors related to this issue. It would also be useful if you could replicate the issue, navigate to System → Administration → Troubleshoot and generate a troubleshoot file. It can be attached to the original post and will only be visible to Teltonika moderators.
Another thing I'd like to ask for is after replicating the issue (keep the NTRIP service running), run the command /usr/sbin/ntrip_client -d /dev/rs232 and paste the output in the .txt file, which can be attached along the troubleshoot file.