Hi
I have looked up your service and these are my findings:
NBN FTTN sync speed is restricted by how far you are from the Node. If you are too far away from the Node you may not see the speed that your plan is enabled for. In your case you are on the NBN100 plan, but you are only getting 50.5M download sync speed and 25.9M Upload sync speed on the modem.
You are 600 meters from the Node and as you can see in the below graph your speed is acceptable for this distance. Your connection is Stable and not dropping out.
This is a physical limitation of copper cabling and no matter which plan you are on or which ISP you choose to go with you will not be able to increase this speed.
The only other possibility for increased speed is to ensure that you have just one phone socket in your house and that it is terminated on or as close to the NBN Leadin cable from the street. Multiple internal phone sockets can have a big impact on sync speed.
You of course have the option of changing your plan down to NBN50 as there is no point in staying on the 100M plan when you cannot obtain these speeds.
Below is a graph of what speeds are deemed acceptable for the distance from the Node.
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I hope this helps you understand.
Regards,
Peter