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Good day @jerh,
Welcome to TPG Community!
Thank you for the information that you've shared with us. Since you are able to receive a good speed directly from the NBN NTD box.
Have you tried to reset and reconfigure your AC1200 router? If it's done. Please shoot me a private message of your preferred time and best number to be contacted and I'll arrange a call from our Tech team to look into it further.
Regards,