Welcome to TPG Community!
We were able to locate your account using your community details and learned that initially your plan is under the Standard NBN12/1 speed tier, which is why you were getting the speed that you've posted.
We understand that you've requested to upgrade your plan to the NBN50/20 speed tier and we can detect that the sync is within specification.
If you still experience a slow speed issue, you may refer to the troubleshooting that we've posted on our article. Here's the link.