I'm surprised you are having problems using the Telstra Max gateway modem.
I have recently been migrated to the NBN (FTTN) and I use the Telstra Max.
Here's some speed checks.................
(1) From my desktop computer connected via ethernet
http://www.speedtest.net/my-result/a/2009125088
(2) from my mobile via wifi using the 5G channel
http://www.speedtest.net/my-result/a/2009125088
BTW......the speed test on the mobile was done in a different room to the modem some 15 metres away.
As you can see, very little difference in speed.
Cheers